I think it does indicate a difference in the way a DiL regards the mother of her husband, regarding who is/not in the family "circle"
I think being a MiL is different to expectation - and for me, what appeared to be a lovely relationship, changed once a grandchild was born. DiL's parents were obviously regarded as family, encouraged to visit and stay, plans made, I later found out, months ahead. My involvement, although I live nearer, just somehow involved occasional fitted in visits after all other arrangements were made - I felt just like a health visitor/social worker in my contact. I had to ask if it was possible for me to see more, but my trust in DiL has gone and I am wary. She may just be thoughtless, but I feel I am not regarded as family in her book and I now feel there is a glass wall between us. My only child, DS goes along with it all and I do not rock the boat as he has his life to live and now it is with her. I am just hurt and am quite sad when I read how other MiL's have a lovely relationship - I thought I had one once.