MrsRyan I salute you for admitting and sharing these thoughts especially here where people can be so black and white and arsey. But also for all you've done and are doing for your dc, your parents who failed you and your ds who has not fault but is hard work.
I know a bit where you are coming from in rel to having to take someone in and being split between wanting and not wanting, between resentment and kindness.
I have done and often I wish I didn't but at the same time I knew I could not do it. I resent, like you, the fact that the shortcomings and selfishness of others has impacted on my life and because, like you, I care about those left behind, I also feel they left me no choice as I would and could not not help. And exactly like you said I am so angry that nobody cared to ask me before hand or thank me or anything. It felt and it was an imposition.
And yes in these moments, which is not all the time, you do think about the money, the holiday, and the rest. Why not?
Anyway there are also lots of positives but those are not a problem.
My suggestion to start with is, do not think to far ahead (like uni fees etc as that is a long way away and will only cause anxiety). Plan ahead but stay in the present, day by day. Try to talk about this with those involved, you sis, your dh, and the ds surviving parent. It will burn less, they may share your feelings and you'll be able to voice your concerns. They then will be on the ball more.
Do no do what I did make it so easy and lovey that nobody realised how I struggled or how tough it actually was for me and I was isolated with my feelings till I exploded. Which will
Not be good for your dsis as you'll resent her because of all the above.
I always think of Harry Potter and now I know how the Dudleys felt. ( lighthearted).
In other words do not make something really hard a walk in the park because you'll end up confusing yourself in the first place.