AIBU to think I can’t fix the problems in my ex’s blended family?

I think for at least a while, you should have your daughter with you every other weekend and do it when his stepson isn't there. Surely that solves the situation for everyone and stops him from being blamed for a problem she didn't cause
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I would not agree to a meeting where the onus is on her to modulate her behavior. The father created the problem by having two children that weren’t ready to cohabitate. Both children deserve to relax and be comfortable in their home. If he rushed the blending he should roll it back and restart.

send her the weekends the little boy isnt there? and then once in a while she goes when he is.

I would not be sending her every weekend - you need the fun bits too!!

I haven’t read all the replies so sorry if I’m doubling up.
This is really hard, and I’m not surprised your daughter is finding it tricky. It’s not her lookout to have to change her behaviour, although she can try to get along with him. From his perspective another child comes to his home and disrupts things ( of course this isn’t her fault or her intention but you can see why this might dis regulate him too) Again, this isn’t your problem but it does involve your daughter and it might help to explain this to her too to try to understand things from his side, he has also had a new baby and new child come into his life.
I would have a meeting to discuss all this, as much as it’s hard they are a blended family and it requires the adults to do the talking for them. Other than that the only suggestion I have is to ask your daughter what she wants, is it too much going every weekend, would she appreciate a little break. Very tough and it sounds like you are being very reasonable and mature in this situation, which is exactly what all the children involved need.

I’d say that for a nine year old to repeat what she had heard from her “sister” was actually very rude and disrespectful to your ex’s DP. No your ex can’t control what is said in your house, nor can you control what your DD says in his. However you can try to ensure she is not routinely rude to anyone.

I wouldn’t be meeting together to discuss this. You would be at a disadvantage.

The obvious solution is that each the 6yo and 8yo each do true 50/50 with their respective sets of parents - half weekdays, half weekends - and for now they are not at the house together. They can be brought together more - for example, for family events - over time. But you are not the obstacle to this happening; all you can do is say you're willing to facilitate this once your ex gets his priorities in order and figures out how to make this happen for his daughter. The rest is a distraction and he's inappropriate to try to dictate what happens at your house during your time with your daughter provided she's relatively happy and not in any danger.

He needs to see her on her own- eg during the week - so she gets to spend more time with her dad minus her step brother. He chose to move - he needs to make an effort to come and see her

It seems obvious she should go alternative weekends, alternating with the boy. I think that is the conversation you and her dad should be having.

OP - watch Bonus Family on Netflix.

It is a Sweedish dramedy that has this kind of family dynamic, and all the ups and downs that it brings.

It is very funny, heartbreaking, really interesting to boot.

Not your problem. Three children with three different women says it all.

Very judgemental comment. Especially considering the father doesn't have 3 kids.

Take your ex out of this for a moment. I would want to have my own DC at least eow (in addition to the week) and I’m concerned you have not insisted upon this. Potentially add in more activities she would love to help make this more attractive, but ultimately you are her mother so make it happen anyway.
Then the onus is on her own father to make the effort to visit her in the week. Many, many people have long commutes of over an hour every weekday so the distance is inconvenient but possible.
He sounds out of his depth, and siding with his new DW, so by default siding with the SS over your daughter.
Honestly, is it really convenient having mostly childfree weekends with your own new partner?

Somewhere in all of this are two very young and currently unhappy children where the adults are all putting themselves first.
It is your issue because by just stepping aside you will allow two other adults to decide how blame is apportioned, and then the consequences/punishments meted out to your child, every weekend and most holiday time.

I would be meeting, but only to be ensuring my child is not being unfairly ‘parented’ and essentially scapegoated. You talk about being potentially ganged up on by two adults but don’t forget those are the same two adults left in charge of the situation every weekend for your nine year old. Are you not concerned about them hanging up on her?

I’m in 2 minds about this because although as you say it seems it’s the dynamics in his house that are the issue your daughter might end up suffering the consequences of their decisions and personally I would prefer to have a say in it .
children those ages argue a lot . Mine are siblings and 4 and 9 and at times it’s like cats and dogs . They do get punished for really bad behaviour and failure to respect each others boundaries but with the oldest it’s harder as she will use it to try and guilty trip us . I’m assuming it must be even harder on your ex as he knows how jealous she is of the 6 year old so probably doesn’t what to constantly punish her .

Spot on. I spoke ne querying what the ex’s new partner is doing to help support the little girl when she’s away from her home and mum and expected to slot in with this new family every weekend as the visiting child. I would not like the idea that he’s essentially saying they’re all fine until his daughter turns up one bit.

Why doesn’t your child go to her dad’s only on the weekends the little boy isn’t around? Maybe see how that works until they are a bit older?

I think I would be suggesting a little trip out with them all, just an hour in the park when you drop her off or something like that. Tell them it will give you the chance to see what's happening for yourself as you don't see the behaviour at home. Tell your daughter it's so you can meet the people she spends her weekends with. That way you can see for yourself if this is your daughter misbehaving or the little boy, or a mixture of both (and possibly stepmums parenting if her little boy can do no wrong). This will enable you to see what your daughter is dealing with at the weekend and if you are able to help her whatever the dynamic is.

My kids are 9 and 7 and they do fight a lot as my daughter finds her brother annoying so it could be an age thing. Be careful that they aren't just disaplinning your daughter because it's easier than sorting the boys behaviour. Although with the dynamic between you and your ex I don't see how that would be fixable. My ex is similar but it's our son who is always to blame.
I'd be temped to go to the meeting with a hard hat on ask what they are doing to make your child feel more included and settled in their home as she is in yours and suggest things that will help. It won't improve your relationship but might draw a line in the sand about expecting you to fix shit you have no control over.

I’d say to dad that he needs to manage this otherwise it will ultimately damage his relationship with his dd. Could she go full weekend on the weekend the boy is at his dads and then do a couple day trips with dad the other weekend and maybe one evening on the week her and dad do an activity together?. Does he actually pay maintenance because he can’t be having her 50:50 based on what you described. And even if he does unless he’s paying half of all expenses- school uniform, trips, clubs, childcare etc he would still need to pay maintenance.