My Dad died, and I moved back home to help my Mom make the transition. I was there for about 3 years. During that time, I married, and my daughter was born while we lived with my Mom. This caused her to develop a much stronger relationship with my daughter than she had with any of the prior or future grandchildren. Even on holidays like *********, she did much more for my daughter than any of the others, and everyone saw it. How bad was it? It got to the point where my daughter resented being the obvious favorite. My son, when he was young, pushed his way in between them to get his share of affection. My wife currently has 3 grandchildren, and we love them all equally, but I don’t doubt that should one of my kids have a child I might have a stronger connection with them, but I will try my best not to let it show. Regarding your situation, Jon’s sister may have poisoned your family to his Mother, or she may worry that if she is kind in any way to them, she may jeopardize her situation with his sister. Have your kids telephone her. Make sure they acknowledge her on special days with cards and such. It may be that she feels forgotten. Or maybe she is just flaky, but you will know you tried.