Even though our son and daughter-in-law aren't married any more, we treat both parents as though they are our special people. When we have a cookout, we invite our grandson and both of his parents. I believe she is still his mother, and still our daughter-in-law. Is this weird, or are we helping our grandson in the long run? It works for us.
I don't mean to rain on your parade, but in my opinion, this would be awkward for everyone; it is for us.
My fiance of seven years stopped talking to his parents about 4 years ago specifically because of this reason. When he split with his son's mother, he felt their relationship should end as well. While I understand that she was there for a long time and bonds were formed, I can't help but agree with him becuase of the issues it has caused.
She was able to use his parents (and everyone else) as a babysitter prior to asking him to take the child - he was losing out on visitation because she really was USING his parents.
His parents would invite her to functions...while he tolerated her, why would he want to see her? They were separated. Her side of the family has plenty of functions where she can act motherly and interact with relatvies, she doesn't need to be included on his side as well. It was even stranger after she re-married and they invited BOTH the ex, her new husband, AND their daughter.
Because she talked to the family on a regular basis, when topics came up like "who was right" or "who felt what" things get hairy. When they went to court she tried to bring his parents is as witnesses in her favor...and for a while they were going to agree, until they realized they only knew her side of the story.
You see, your children don't tell you everything. Even if you think they do, they don't. So while it may not bother him on the surface, it probably does deep down.
Had his family cut her off years ago, we wouldn't have had to spend thousands of dollars in court against her a few years back. And we won by the way...so now where does his relationship sit with his parents? Now it sits in one of those, "I told you so" positions where his parents hardly talk to either of them and rarely see their grandson...or our daughter, their granddaughter.
If it works for you (and they thought it worked for them), that's great, but please, be careful that you are not jeopardizing your relationship with your son and grandson because of feeling the 'need' to bring everyone together. Their past is the past...she should not be offened if she is not invited. She might even be showing up to secretly 'twist in the knife'.
Good luck! Life is complicated.