Just let her come to the party, be polite, and say nothing about her previous behavior.
Yes, this is about your son!
Don't waste your time trying to figure out her behavior....you cannot change it.....but you can control your own, and give your children the example of polite behavior.
IF she is NEGATIVE, walk away.....just say "I beg your pardon, this is a party/celebration for (insert name)" or "NO negative behavior here!", and that should go for any family member.
Always invite her, she is the grandmother, be kind and polite, but do not tolerate negativity.
So what if she sits 10 rows away.....that makes it easier on you.
If your children ask about her behavior you can truthfully say "I don't know...but she is still Grandma, so be polite."
She might someday get it,,,,,,but maybe not, you never know.
You are probably getting a glimpse of why she has had several marriages!