It sounds to me like your sister knows you have a lot of responsibility and I don't think she wants to contribute and get involved with your situation and caregiving.
It may be her way of distancing because she has made a decision to not allow herself to be responsible of your father and brother, their mental issues and autistic niece.

I don't think that she is purposely abandoning you so much as she has made a choice to take care of her personal happiness. So when she talks to you about her family issues, well that is ok. But as soon as you talk about your struggles, well her silence is a combination of guilt and that she doesn't want to accept any responsibility for what you are stuck with. Sorry, but that is how I see it. She got married and booked and she has no intention of getting involved with the father and brother and their issues.

Honestly I cant blame you for being stressed, you have a lot to look after. I am glad to see your connected with a church or bible group, maybe you need to reach further than you are into a bigger church or something. What do you have for you, it seems your life is about caregiving to others.

((((Hugs)))))

I am glad you found PC, you do need a place to vent and connect and get support.
Open Eyes