My kids are part of their siblings’ parties until/unless they don’t want to be. So far, they have always joined in except the one party that involved playing a sport the other one doesn’t play. Inviting a friend for them is fine.

Mine are 8 and 5 and up til now they have always wanted their sibling at their parties. We’ll see how it goes as they get older. At a lot of venues, there is enough room that they can go off and do their own things without having to be around each other that much.

My oldest had enough of that when he was 12. The parties get smaller and it's weird to have younger siblings there. Also, older sibs get tired of the little ones parties, though I did pay my older DS and a friend $20 each to be "helpers" and my youngest's trampoline party a couple years ago. (DH was sick and none of the other parents were planning on staying. I didn't need any help but was glad to have 2 responsible tweens about in case someone broke a bone and I was fully preoccupied.)

Talk to your kids about it and take their lead.

I have 3 kids - ages 10, almost 8, and almost 5. I like my kids to attend their own siblings bday parties. Is that weird? What do you do?

I have started letting my other kids invite a friend??