That's what I was thinking. She loves that you take both of her kids.
We have several kids in the neighborhood all around the same age as my kids withing a year or so. That said, my daughter who will be 10, has different interests than the girl who is a year older than my youngest. This girl likes to play with my older daughter, but my oldest gets tired of this kid after a while and wants to play with her friends who are the same age. The mother if the kids understands, the father- he has a complete fit that my older one wont play with his kid. Even though there is a huge amount of other kids HER age incl my youngest.
Just tell the mom, your older son is not interested in playing with a 7 yr old and it would be more of a sitter situation and since she dosent want any male sitters, dont send him over. This mom could be at her wits end with this child too since he no doubt has trouble with other kids. Maybe she thought an older boy would be a good example but that is of course not fair of your son unless he is game but he is not.
The girls are 9. The brother is 7. The problem with girls including him is that he is difficult. Mostly, he hits. My daughter doesn't even like to go play at their house because she gets hurt so often. Or he ruins whatever they have been working on building or creating. Rips the doll clothes. That sort of thing.
I would think that at 7 he should be able to handle his sister going down the street to play, but maybe the age difference is at play here? My kids are 3 1/2 years apart, so this was almost never an issue for us.
I don't think your son should be held hostage for a playdate. But, I have similar issues going on with ds2, right down the behavioural issues, and the unwilingness of other kids to play with him. He's been tentatively diagnosed with ADD, but I suspect something like Aspberger's Syndrome. He has anxiety issues, which took me a long time to figure out, because of the way they manifest. There are about 8-9 kids in the neighbourhood who come by wanting to play with dd1. There are two who come by to play with ds2, and one of them only does so if everybody else (including dd1) is already busy. DS2 can't handle his sister going down the street to play, because he feels chronically left out, rejected and helpless.
I don't agree with the way the other mom is handling this. But, I do sympathize. It's hard to watch one of your kids take off for playdates and sleepovers, while another child is at home hitting himself in the face, and saying, "I'm a jerk and an idiot and nobody likes me, and I don't blame them, because I don't like myself, either". It's heartbreaking to see ds2 light up when someone comes to the door, only to have that person ask for dd1, and then say, "no, thanks" when ds2 says "I'll play with you" with a big smile. I can't make other kids play with him (and I even understand why they often don't want to)...but I understand the temptation to try.