Sister not offering and help?

Well my sister lives only one hour from my Mom and Step-Dad's house she has promise to do some premade meals and baking but never did. I have been here for one year taking care of them both I was only supposed to stay for the winter but my Mom begged me to stay longer I once ask my sister if she could come help because my back was sore she blew up and said our Step- Dad's family can do the helping now I am suffering from care burnout but could never tell her this because she will say oh well put Mom in a home etc Last year she was fostering a child but now the child was adopted out she never asks how I am doing rarely answer's my text's I send her update text's about my Mom now last night she sent me a text picture of her and a little boy saying look at our new addition to the family and I was so hurt I know she is only doing it for the extra income some of you might disagree but I know my sister it is all about money and on top of it all she is in denial of her drinking? I could not sleep all night because I was so mad and hurt I want to tell her how I feel but my partner said what for she will just twist it and say well it was your choice to look after Mom etc it is bad enough I am dealing with my manipulative Mother and Stepfather but now this how should I deal with this any advice yes I am getting support caregiver counseling and in the procedure of having a caregiver come in to take care of my Mom and Step Father any advice?