I see so many families dealing with similar circumstances so don't feel you are the only ones.
On my side my sister-in-law and brother live less than five minutes from my mother. I live 16 hours away. My brother loves my mom dearly, but my sister-in-law is so hateful toward her and resents anything my brother does for our mother. She tries every chance she gets to exclude my mother from anything. I could go on and on, but it hurts my mother deeply that her daughter-in-law is so hateful. None of us can figure out what my mother has done to her. She's turning the grandkids against my mother as well. That cuts my mother deep. My brother won't do much of anything because their marriage is so unstable. So she has a emotional strangle hold on everyone and can do whatever hateful thing she wants because no one wants to challenge her and thus give her any fuel to use against us. It is a very sad situation.
Then on my husband's side we have a sister-in-law that does so much with her family on a regular basis, but I bet we don't see them but a handful of times a year. It is a major feat to get them to do anything with us. With her husband's immediate family and especially the extended family. My husband and I live one street over from them and you would think we lived miles away. We see them less than the neighbors. If you invite them over, they may or may not show up and they are sure to leave very early because they already have plans to do something with a neighbor or her family. My husband would love to move back to the other side of town to be closer to his mom/dad and other sisters if we could because it hurts him that he lives practically next door to his brother but never sees him. I could go on and on with that as well.
Basically we have learned that it is best not to say anything, insinuate anything, or do anything to make things better...because it always makes things worse. It is their choice to exclude us for whatever reason they are not sharing with us. It is their choice to disregard invitations or only show up for a token visit. It is their choice not to share anything about their lives with us. It is their choice to keep their children from developing relationships with us. We make the invitations and leave it at that. At no time will they ever be able to say we didn't try.
just be polite and keep trying to get them to visit. If they do, great. If they don't, they don't.
Maybe you could develop a friendship with a neighbor with kids and spoil them instead. Maybe join the Boys and Girls Club and pour your grandparent skills out on kids that don't have a good family.