My son's 9. One of his besties is 13, and one of his cousins is 13. He spends a lot of time with BOTH. He absolutely doesn't go to their sleepover parties with friends their own age. It's inappropriate. The age difference is HUGE. They would have to make the party all about my SON. The cognitive and emotional development is very very different. On an individual basis, fine. No problem. But send my son in with a group of teenagers? No matter how nice the teenagers it would NOT be fair to them OR my son.
It's absolutely no reflection on them or my son... it's just a reflection of age. They're young adults and my son is a child.
Was he sad when the split first happened? Of course. So I cheered him up and explained things to him. And he got it, pretty easily. And since then, he might be a little disappointed that he's not old enough, but then he's on to other things.
I certainly wasn't hurt or angry that his playmates were growing up. And quite frankly the sexual education that happens with whispers or crude jokes and stories once parents have gone to sleep wasn't really one I wanted my son having. Nor the movies they were allowed to watch. Nor the group 'angst'. Nor many of the other things that are the purview of teenagers. It's a bit of a problem in our lives, because my son is "gifted" he can talk programming or Count of Monte Cristo or other school matters with them... but he is NOT at the same maturity level they are. He's a child. They're not. It would either be forcing him to grow up too fast, or forcing them to be 'babyish' and resent him for it. No thank you.
As to your SIL not taking advantage of inviting your daughter to the party so SHE could spend time with her during her own child's birthday party... I think you're just upset about the other thing... or you'd know that's a bit silly. Whose focus is YOUR attention on at your child's bday? Right. It's their special day. Not a day you spend with someone else instead.