To have said no to this request RE: DD's birthday party?

[muminthecity]

DD is turning 8 next week. Money is very tight at the moment so we are having a party at home, on a shoestring budget. We live in a 2 bed flat so space is an issue as well. DD has invited 12 children from her class, I am expecting 8-10 to turn up.

One of the girls invited has a younger sister in the year below. DD knows her but isn't particularly friendly with her. Anyway, this girl's mother sent me a text in reply to the invitation saying "Hi, thanks for the invite, X is really excited about the party, but as the girls are too young to understand that they don't always get invited to the same parties, I can't possibly bring one without the other. Is it ok if I bring them both?"

I was a bit taken aback by this tbh, the girls are 6 and 7 so not babies and surely at that age they should be able to understand that they can't always go to the same things? Also, many of the other children invited have younger siblings who are not invited.

I replied to her saying "I'm so sorry but I just don't have the space to invite siblings." She then replied saying that her DD would not be attending as she would hate to upset her little sister. She also mentioned that both girls were "very disappointed" with a sad face at the end.

So, AIBU? Should I have just said yes and let them both come? I'm feeling guilty now that X has to miss out, but also think that it would be unfair to invite her sister but none of the other siblings. Not to mention the space/money issues!

[3birthdaybunnies]

YANBU - slightly different if they were 2 and 3 and couldn't leave 3yr old on their own, but at 8 there's no chance that she will be hanging around. She's missed an opportunity there to have nice 1:1 time with her younger daughter and maybe buy something which doesn't match her sister's. My dc enjoy it when it is 2:2 instead of 2:3 and there's often cake involved so I enjoy it too.