This is not really an AIBU so apologies to the thread police 
It's more of a WWYD.
Been with OH 8 years two of his kids we have EOW and about half of the school holidays. I love them very much and they don't remember me not being with their dad.
I have an ok relationship with his ex which deteriorated last year after I expressed concern over the state of the house and various dubious lodgers after I visited. This has been drawn a line under but we are no longer friends.
OH and ex split up when the kids were tiny, there were a lot of explosive rows and things thrown by both parties (both have told me this) and once she threw a bottle at him and he grabbed her round the throat (I am not condoning this). He went to counselling and they split not long after (they should never have married IMO).
She has always maintained that he was abusive and she felt she was in an abusive relationship to all of her friends and family and all of her susequent partners meaning that the reception by any partners to OH is always frosty (OH is not and has never been abusive towards me).
It's come out this weekend that she has told the girls (aged 10 and 12) numerous stories about their marriage and his "abuse" towards her. A lot of these stories don't tally (one involves both girls when DSD1 was 18 months old and there are two years between them).
Considering that he is "allowed" to have the kids whatever did or did not go on in their relationship is IMO in the past, not relevant to his relationship with his kids and totally inappropriate (I think mentally abusive) to be relaying to the girls (I never ever did this to my own kids about their dad).
Problem we have is that whenever OH has tackled her over something the girls have said the girls then get into trouble (and she totally ignores whatever the issue is anyway so it's a waste of time) so he can't tackle her. DD1 in particular confides in me a lot (esp since the most recent boyfriend has moved in who she hates) and I never betray her trust as I think it's important that she has people here in our home she trusts and feels are her allies (ideally everyone in both bloody homes!).
I feel so uncomfortable about this, I suggested to the girls that they just tell her (politely) that they don't want to know but am not sure what else we can do? Nothing? Or is there something else?