I think SS is hurting my baby

Except that there's no expectation, according to MN, for "mama bear" to protect her cubs if it's a blood-sibling attacking them. Only if it's an outsider.

It's why I don't believe in blended families. Too much "mama bearing".

I see what you mean, but I had one or two instances with my own children, I shut that nonsense down quickly and they are as close as can be now. There is no excusing violence between siblings, at all.

I don’t see anyone excusing his behaviour. They are trying to find reasons for it, not excuses. They are different.
OP has already says he acts this way when he doesn’t get attention. Coming down on him like a ton of bricks is likely to make him feel worse and he will act out even more. Not a great outcome is it?

The way some people are talking about a little child who has, and continues to have a chaotic and tumultuous upbringing beggars belief

He really needs to be assessed for ADHD IMO, going by everything you've said.

This is a good start. But actually if you suspect ADHD I'd just go more in on this.

For the rules - make it really clear and break it down more/provide more reminders than you usually would.

Reward charts great. Exercise great. Diet is probably not going to have much effect if it is not being followed on the other days, so I'd probably go with making sure that he's eating enough/little and often rather than worrying overly about it being healthy. My 6yo (diagnosed ADHD) really needs prompting to use the toilet regularly as well as eating because I think he doesn't pay attention to his body sensations so he doesn't know that he needs to go, but it will put him into a rotten antagonistic mood if he does and he is ignoring it. We actually have trying for a poo as part of the bedtime routine which has helped a lot.

You want to rely much more on positive reinforcement than negative. And curiosity I find helps a lot too. The problem is that if he is doing it out of some kind of desire for revenge or because it brings him a sense of power or control, then the more you come down on the behaviour and shame the behaviour, the more he is going to want to do things which bring him that sense of power or control.

I think this will be difficult to do when you have the relationship that you do - when DS2 (the ADHD 6yo) was 3 and his brother DS3 was newborn, he bit his fingers a few times. Not hard, but hard enough to hurt. Because they were both mine (? maybe?) it wasn't too hard for me to remain neutral and approach it with a curiosity angle rather than getting angry at DS2 or punishing him. Once we had talked about it, all I had to do was say something like "I'm worried about your mouth being close to his hands. I'm worried that you might be thinking about biting him." and it helped to dissipate the feeling and maybe also the urge. Sometimes children (especially children with ADHD) do something just because the thought enters their mind and they want to see what will happen. I think that can be true even when it's something they know that adults won't be happy about and deliberately hide from them - it's not purely an impulsive thing, it can be curiosity over a longer period of time. So if you can possibly approach it from an angle of - don't give him a chance to lie about it, just present him with the information that you know, and let him know you're not telling him off/he's not in trouble as such, but you don't like the baby getting hurt. You know he likes the baby and he likes being a great big brother so what's going on, and could you figure out a plan together to deal with it?

OTOH I remember a longer time ago when I was helping my SIL out with my nephew, helping him with his homework which was tricky for them to manage and he seemed to respond better to me being his aunt rather than his parent. But then he did something to hurt DS1 and although it was totally unrelated I was just so furious I could not bring myself to help him any more.

Of course a stepchild relationship is different to both, I'm not saying this to suggest you are like an aunt, just that it can be really hard to do the neutral approach when you're feeling so strongly protective.

Another helpful thing to do BTW is to encourage him to do things which will give him a sense of autonomy and competence, so basically encouraging him in any interests/talents that he has, giving him responsibilities (e.g. chores) etc. Autonomy and competence is most likely what children are looking for when they are engaging in things that seem to be a power/control thing.

Re the baby not crying-

I have 2 very fair, very light skinned children who as babies/toddlers bruised very very easily. We used to say "like a peach" I once left a bruise on DC wrist when pulling him back from running in the road and he barely registered it but left very clear finger prints. I was so upset.

But I just wanted to point out that just because baby didn't cry doesn't mean it didn't happen.

I would absolutely not give him another opportunity alone with baby at all.

And it doesn't matter if he is step or not-he is hurting the baby.

I'm not excusing it at all, hence why my post states (more than once) that either way it still needs dealing with. I was just expressing a trail of thought/possible situation.

Well what can you expect really ? A mama bear will protect their cubs . Step siblings or daughters / sons etc are not blood, it makes a big difference . Usually the father is more than useless in these situations.

Step mums are berated on here if they try to help the step children and treat them as their own( aka discipline ) or if they don’t.

Personally if these were both my own blood children I would separate them .

I thought that, it's why I hesitated posting, but the things with the siblings seems different almost, more like general attention seeking/sibling rivalry kind of behaviour?

I'm probably way off base and either way it needs addressing of course. I was just trying to think of a different angle, I guess.

It sounds like you are quite supportive of him, so work with the school. Tell them about his behaviour at home and your concerns over social interactions. The class teacher can monitor this (they might already be doing so) plus make sure he is engaged in topics like PSHE where things like empathy and emotions are dealt with. They might also have groups for children with social needs or can take him out for 1:1 time and intervention. I would suggest it's better to raise any concerns sooner rather than later to keep track of.

OP, what are you going to do about it?

I find the opposite. MN is really complacent towards blood-sibling violence (I've posted before about being battered by my younger siblings and been told it's just what siblings do, they hated being an only so would've loved a sibling even if they were "a bit rough", that you can't expect parents to keep an eye 24/7, that I'd be thankful to have siblings in old age etc etc etc), whereas when it's a step-sibling doing it, it's all "go mama bear, protect your own child, banish the little shit for all eternity and fuck anyone who says otherwise, even the kid's father".

You absolutely are excusing the behaviour . OP needs to protect the baby , whatever the needs or intent are. As above he also hurts his siblings .

That would be a good situation to consider, but he also tries to hurt his older siblings which makes the ‘baby smushing’ quite implausible for the reason.

You'd think the baby would cry out in pain if someone was causing bruising? I'm not saying it wasn't him but I'm just surprised it wouldn't have been heard, even if not seen.
Definitely don't trust them with the baby but you already know that. Maybe the older one needs to speak to a counsellor or therapist? If they're doing harmful behaviour.

@GreenPinkLilac Oh god this whole situation sounds really hard, for everyone. You sound like a really loving parent and step parent op.

This might be totally wrong, but I'm just thinking reading through...

It sounds as your SS very likely has some form of additional needs, have you ever noticed any "sensory" type behaviour? I only say because if the baby wasn't crying in either instance (and you mentioned she generally isn't one to cry much) could he be squeezing her and genuinely not realising he's hurting her?

Babies bring out a really strange "smush" response in some people - like an urge to (not violently) smush their little faces or "gobble them up" or squeeze them.

Obviously this is something regulated adults know not to actually do and I'm absolutely not excusing him hurting the baby/leaving marks. This is serious and needs addressing.

I just mean, if he's a young kid with some additional needs and the baby wasn't obviously in distress, is it at all possible that although he's absolutely acting inappropriately, that it may not be "malicious" so to speak?

I agree with this . It also always gets excused if it is a child from a first family doing it( on mumsnet that is). Mumsnet hates step mums .

@GreenPinkLilac Sounds like stepson is a hot potato child, passed from pillar to post with zero consistency and good strong boundaries.

Far from ideal.

Sounds like he has no solid grounded home where he can relax abs feel safe , hence the attachment to you and jealousy of your children.

You have to prioritise your little one OP . Don’t listen to the poster trying to gaslight you .

Are people really excusing this behaviour because he is 6? I have a 5 year old and I would come down so hard on them the moment I found out that they had purposely hurt a baby (mine or otherwise) that they would never even consider doing it again. 6 year olds understand what they are doing ffs.. this isn’t acceptable behaviour no matter what changes they have gone through.

It's not you we're talking about though is it.

The 100% only solution that will be acceptable to social care is you being vigilant and keeping them apart.
Sorry. It will be their view that you are the adult & must be responsible.

Invest in some cameras, both to confirm or disprove your theory, and for insurance. I've seen friends get into a lot of trouble with social services when they didn't know how their child was injured.

Seems like a great way to live . I couldn’t live like that .

This is very difficult exactly because he is not in your home enough for more structure to have impact. I would say, that when contending with a very permissive mother, even 50:50 isn’t enough. I know through experience.

Your husband needs to at least try to set aside the acrimony with his ex and discuss their sons behaviour, agree on how they will address it. I’m not suggesting all the acrimony stems from him, just that this is serious enough that he must hold firm when dealing with her.

You continue to prioritise your baby and keep her safe.