That does sound really hard @Vie8126.
To an extent I agree with @sassbott - particularly about how to handle it with your children who will understand that your SD is treated differently. In fact, they will probably reflect to themselves on how the Disney dad inconsistency is not good for her in the least. I'm know that my DSes (12 and 21) look at the SC and think it im ridiculous that their father doesn't sort out several behaviours.
Where I would differ is in relation to the baby issue. I think that it is probably important that you talk to your husband now about what's happening and sort something out before the baby is born/before it starts to feel urgent and high stakes. It would probably be good to find a counsellor to help with this.
I say this because my own situation looks a lot like many parts of yours. And, contrary to the passing remarks in so much of the stepparenting literature that having a baby together can often improve things, my own experience has been entirely the opposite. The need for the baby to have consistency in his life and to not copy his half sibling's dreadful behaviour is very important. Things I could explain away to my DSes as just different not unfair, just cannot when there is a younger child who is seeing his father let some children behave dreadfully (and undermining the rules to which he will be subject).
Particular flash points we've had are around:
- The absolutely dreadful values the SC are being raised with which come through in their behaviour. Their mother is awful. And she is bringing up rude, disrespectful, demanding children who are encouraged to see any situation in terms of what they can get. The way in which the SC talk to everyone, including each other, is unacceptable. And it's not OK for the baby to be subject to the horrible ways they interact with each other. SD is horrible to her brother, and I am not OK with my son being subject to being told how much better she is than him and everyone else at all times. She routinely tells SS how she's better in every way than him and it is not nice at all. SS is aggressive, attention-seeking and really unpleasant. Already he does things like push the baby (who is cruising), shout 'get off' in a nasty tone while doing so and snatch things from him while shouting at him or purposefully scaring him with aggressive behaviour.
My husband's inconsistent, guilt-driven Disney parenting is making all this worse and it's not OK. Their behaviour had to improve, or their interactions with the baby must be very limited and heavily supervised. Plus I need to be able to trust him that he is capable of stopping his son throwing wooden bricks at a scared and crying 8 month old baby (at the time), pushing him over or driving ride on cars aggressively at him while shouting and terrifying him, rather than deciding to ignore it and write it off as 'excitement'.
Even when the baby was tiny, the ways in which my husband allowed the SC to behave had very negative effects on him. He was continually woken up (and it was a real struggle to get him to sleep) by screaming, crashing around and throwing things, and fighting even from the day he came home from the hospital. It was horrific and very much eroded any goodwill I had towards the pretty much feral behaviour my husband was allowing to distress both me and the baby. It was really traumatic.
- Eating. I've written on this forum about the awful eating situation with my SC. This has become an even bigger issue since the baby was born, and particularly once we started weaning him. It's not OK for him to be learning to eat alongside a whole range of dreadful behaviours. It's not my fault that their father has spent years undermining every attempt to improve the situation to the point that his other children just cannot eat with us. Their behaviour has negatively affected my middle son's eating behaviour in several ways even though he's older than them. The baby needs to be able to learn to eat in a reasonable context rather than the complex power struggle my husband has allowed to develop around his other children.
My husband knows that his children's behaviour is awful. And he is absolutely aware that he does not want the baby to copy or adopt these behaviours. But his divorced dad kicks in and he minimises and fails to do anything other than make the situation worse. It's way beyond 'a bit of fussy eating' or table manners or any of the other things that you can manage with a baby at the table. It's an enormous mess and we are seeing a counsellor. I think his children should have some therapy but he won't accept that (not least because his toxic mother keeps banging on about mental illness running in his father's family and the dodgy genes). But their food related behaviours are so problematic and weird that they really should be seen through CAMHS.
- The dreadful stepfamily dynamic where he does his guilty divorced dad thing, projects everything on to me and he, the SC and his awful mother all use me as a convenient scapegoat. This is awful. It's ruining my life. But it is absolutely not OK for my baby to grow up in a context where no matter what I do, his mother is cast as the villain and the SC are prioritised, favoured and allowed to behave in ways he will not be. That's not just because I will ensure he has rules and boundaries and routines but because his father will too.
You see, the divorced dad guilt doesn't apply to the baby. He can be treated normally because he's just always here. Indeed, he can be deprioritised in various ways because he doesn't need to be compensated for his parents getting divorced.
It is just toxic and the baby is very clearly getting the message that he matters less to his father than the SC (and to his grandmother and aunts and uncles). He's only 10 months old but he will be picking things up. And increasingly so as he gets older.
Based on all that, I'd say that dealing with things NOW is most definitely not borrowing trouble. It's actually trying to avoid it.
It's hard to see how my marriage can survive this. Or how I can have a positive relationship with SC whose father creates such problematic dynamics around. I can't even disengage (as I would if it were just my older children) because I do have a responsibility to protect my baby from all this bullshit. It's not some irrational, maternal protectiveness; it's a hideous situation and my husband needs to address his divorced dad guilt (plus a legacy where a lot of this crap is driven by him trying to save his 9 year old self from his parents' divorce and a childhood characterised by parental alienation, and angry martyr of a mother, a detached father and a convenient scapegoat in his stepmum).
It's worth getting some relationship counselling - specialist counselling about blended family issues - and sorting things out now so that you can feel comfortable and confident with your baby in your blended family. There won't be one right way to organise things, but you can find something that works for you.