I have to respond because this is one of my pet peeves. I budget money months ahead of time for my son's parties. And I purposely write on each invitation who is invited to come -- some households, my son is friends with both siblings. Others, he doesn't know that sibling at all. It is HIS party. So I keep that in mind -- who he would like to be there. And we have to carefully not invite others because that would make the party OVER BUDGET. So there are friends that my son may know but we can't invite b/c of $$ reasons.
Then, the party day comes and some families show up with siblings. And never say a word about it to me. So naturally, the business that wants to make more money -- writes the siblings down on the guest list.
Have you had parties for your girls at kid venues?? It's so expensive. We have so much fun, so that's why we do them. And we plan to spend a certain amnt of money, but I think some people who don't give parties don't realize how much is spent and they bring their sibling kids along rather than just be a parent and tell them 'no, this is a party for big kids -- big sister was invited. you'll get to go to a party later.' It's a life lesson we all have to learn -- we can't always get what we want.
So my opinion is to only bring the child that was named on the invitation. That's party etiquette. And that's what the host is planning on. If you can't find other babysitting arrangements, then you can decline the invitation. Sounds like you guys go to plenty of parties anyway. You can send a gift later to the child, if you want.
Can you imagine if every child at birthday parties brought their siblings along - some have 3-4 in a family?? What a big expensive party! Not very considerate of the party host and their finances.
As for gift ideas, I try to buy a $10 gift item at Walmart and then add dollar store coloring books/paint books/other little toys/activities sets to the gift bag.