Reb..i hope you don't think i'm lecturing you..because that's not my intention. I've been there too, stuck in between my kids and my H. My H never really knew/knows how to handle them. I was never impressed that he chose to handle things differently, or was actually quite indifferent to them at times. My daughter, 18 when this first started, took same path as your D15. She didn't want to have anything to do with her dad and ignored him most of the time. I had my older S24 living at home then, he would attack me and would say I deserved everything I got and Dad was right in what he was doing. The youngest S17, well his loyalties swung between both. It's hard on them, there is no doubt. They are hurt.
My DB coach suggested I DB my kids too. She meant, I had to look after myself No. 1, ignore their rantings and ravings, set boundaries, protect myself, show love, don't try to fix everything for them, and most of all don't let them get in between H and myself. At the end of this they will have a better understanding of the dynamics of a R. At the present time they are too young (even at their ages) to really understand the implications of a D.
Its a really tough place to be, you feel attacked on all levels. I've realised I have no chance of keeping everyone happy, and I don't even try any more. You will never (and its not your job) to keep H, your D's, your MIL, or anybody else happy any of the time. You're a good person. That's all you need to know. I just try to model good character, peace, and love in all I do. They will get it in time. D and H are already restoring their R somewhat, and they are doing it without any interference from me.
In any event, I don't get on with all members of my family, and probably never will. Its just the way it is and I've accepted it.