I would forget about your sister and focus on your mom. You could encourage her to visit but she has to come to terms with things in her own way. People grieve differently. I know when my mom was dying one brother wouldn't come up to see her and stayed away, one brother didn't like to come in the same room and avoided her, my sister wouldn't accept that she was dying and was angry and shocked when she finally died, etc. I am sorry to here about what you are going through with your mom. At least you know that in the end you were there for her and did all you could for her while your sister might end up wishing she had done more and had been there for her. Hugs to you.