I'm going to have to accept that it's not my responsibility to foster their relationship at some point, but the idea that my child has to deal with this is a bitter pill.
I felt that my ex was like yours for a long time. I felt like my kids were lacking in attention and care because my ex seemed so focused on the R with the AP and on the D. Over time, it's leveled out a bunch; though I feel like their relationship could still be better, Ive accepted that it isnt my job to police that. And frankly, I imagine any efforts I had to do that would have resulted in the opposite effect.
Im sorry your D is going through this. No child should have to suffer like this. All you can do is make the best of a bad situation.