Both my sister and I are DF'd although it was years in between. My sister is now 40 and the mother of 2 children... I am 30, married with no kids.
My JW mother has visited and even stayed in my sisters home because she wanted contact with the "innocent" grandchildren. She visited with her for weeks and sometimes months at time over the past 8 years that the children have been in the picture.
I live 20 miles away and have been DF'd for about 5 years and she refuses to talk to me, hang out, have lunch etc. She refused to come to my wedding even though it was held outside and had no religious theme.
I was very conflicted, very hurt, felt deep feelings of abandonment for a long long time. THEN I finally understood and resolved it all in my heart. My mother can refuse to speak to me, but I can also refuse to speak to her on the principles that she has been a mean-spirited, uncaring, unloving mother. She might not want to talk to me based on the fact I am DF'd, but I refuse to be friends with her based on her ACTIONS. I did email her before my wedding and we exchanged some verbal crap and I finally told her that I was a PACKAGE deal. You're either IN or you're OUT. You either want to come to our wedding, speak to me like a human being, or you have made your CHOICE to be out of my life and that decision will stand. When we have children, she will not be visiting them, she will not be talking to them, she will not have contact with them and it will be my CHOICE, because I will not have a flip-flopping grandmother in their life. Someone that might love them until they don't agree to not celebrate a holiday or something stupid at which time she will vindictively reject them.
At some point you have to make the choices, you have to call the shots, I refuse to be the receiver of her occasional nod in my direction. She either calls or emails me when she needs gas money or assistance with home repairs. I finally took a stand on that this year, I told her we'd be happy to come help her paint her house, but we would do it like a family and eat dinner together and maybe go to the store all together. She refused, so I declined assisting her with her painting.
When a family member only will speak to you when they want something from you, or as free slave labor, sorry that's not good enough for me. I'll pass.
It sounds like your sister is taking a stand. It was much more difficult for me to try to placate my mother and toy around with her, and have EXPECTATIONS and then be faced with cruel DISSAPOINTMENTS. Once I made up my mind, I've been a lot happier and free of the mental torture and heartache. Good luck to you. I truly sympathize.