Sister has been out of the picture for 20 years now wants to visit. Any advice?

Mom is 90 and had a heart attack on Monday. She came home yesterday (Thursday) and Hospice enters our lives today. She has lived with my husband and I for nine years. My sister (66) chose to remove herself from my mother, brother and my lives shortly after my father died, almost 20 years ago. She has not seen my mother during that time. Sister's phone calls to mom are infrequent. Mom has a separate phone line so it is not because sister is avoiding me. She sometimes will send a card or small gift for birthday or Christmas, but usually not. Notes on cards are always brief and impersonal. She will address it "Dear Mother" and never signs her name. She will sign it for the city she is living in, let's say "Love, Chicago" This is my sisters third marriage. Her children are adults now and they never had contact with thier father or his family. Sister would not have stood for it. When my brother was diagnosed with a terminal illness about five years ago sister went to see him. My brother said she aired all her grievances with him about the family, said her goodbyes and left. She did not attend the celebration of his life. She did call my mom (then 85) and tell her she should not attend his service. She has made many hurtful calls to my mom, but this was the worse. Mom and brother were close. When mom had her heart attack on Monday I did not call my sister right away. I knew she would learn of it from another family member. Yesterday, I was told she and her son (43) were talking of coming to see Mom. I asked my mom how she felt about it and would she like them to visit. I told her there was no wrong answer and she could think on it. She thought on it and decided she was okay with them coming. I called my sister last night and nicely told her I heard she was th