PLEASE don't take this as me putting more pressure on you. I am so astounded reading this and admire you very much for taking this all on in such a big-hearted way.
My daddy died of a brain tumor when I was 16 (coming up to 4 years ago). He was diagnosed 2 years before he died, and their diagnosis was that he had less than 6 months to live.
My mama nursed him at home through the whole thing (apart from two hospital stays for operations, + chemo & radiotherapy).
During this time I experienced the "pulling away" that you mentioned in relation to the 12yo. I not only found myself distant from my daddy but my mother as well. At the time I was dealing with high expectations about my schoolwork (I was always some sort of class leader) from myself as well as other people, amongst other things. This sounds somewhat like how you describe your Dsd.
At the time I guess this pulling away (particularly from dad) was almost nesacary for my emotional survival - I went on with my life, making as little concessions to my situation as possible. However, now I really regret that I didn't let myself be "different" for a while & spend more time with my dad while I had him. And it's an awful regret to have.
SO, while it may make you feel like the bad guy in the situation, please know that your efforts to keep your Dsd involved with her mum, and the support you are giving these girls through this awful time in their lives is absolutely the right thing. You are effectively making sure they won't have this regret.
Even if they occasionally resent it now, it will be so important to them later in their lives that you were there for them & helped them be there for their mother.
You have probably thought about this already but I just wanted to reinforce it & remind you how much difference you are making for these kids.