DD1 (turning 7) is having a small birthday party at our house. I'd like to keep it at about 10 guests, including her and her younger sister.

I had told DD2 (5) she could invite one friend, who is the sibling of one of DD1's guests, to keep her out of the big girls' hair. DH then told me that he thinks we need to invite the younger sibling (also 5) of another one of DD1's guests because "we always do things as families with them" (I totally disagree, but whatever). So that brings the total of 5 year olds to 3, and 7 year olds to 7. Fine.

However....a third 7 year old guest really needs to be a girl we will call K. K and DD1 have known each other since preschool, and though they aren't in class together thisyear, they will spend a lot of time together in summer camp, and will be together at the same "neighborhood elementary school" for grades 2-5 (our schools break out after 1st grade). DD1 was just invited to this girl K's party, where there were only about 6 kids invited (no siblings).

However, K also has two little sisters - DD2 is good friends with one of them (5), and the other one (just turned 4) really likes DD2. All three of my girls recently went to K's littlest sister's party (big party venue), and K and both her siblings recently came to DD2's 5th birthday.

So. I think we should invite K (and DD1 is happy to do so), but I'm torn on asking her little sister(s). The 5 year old could hang, but I think the 4 year old is too young for most of the activities I have planned for the 7 year olds, and having both of these girls would mean we had 5 little girls, and only 5-7 big girls. It doesn't seem fair to DD1 to dominate a small, in home party for a 7 year old with that many 4 & 5 year olds.....For reference, it's a sleepover-themed, evening party.

Thoughts? Am I over thinking? I was planning to talk to the mom; the only reason I feel kind of jerky is because it's not like I've said there can't be ANY younger kids...I"m just trying to limit it. K's little sisters will definitely know about the party. I could invite K's 5 year old sister and not her 4 year old sister, but I almost think that's meaner....help!