and stick to them.

You dont want to become the hen mama for all of them.

So make it clear from the beginning that pool time etc has times to invite friends, and times just for your family. Since her kids are hellions also make it clear that she comes when her kids swim.

You need to enjoy your summer and not dread it.

It's wonderful that you've helped out with your BILs boys and sounds like you have a great relationship with them, and them with you and your younger kids, etc. Don't risk something that is working well by saying yes to something you might regret/don't want to do. I understand about feeling badly for the kids who don't/can't come over, but your BIL and his GF need to figure out how to manage their large family, expectations, activities for the kids. They can't force you to stretch your goodwill even further and don't let your own guilt do that either.

If her kids are well behaved and she manages them well, maybe you'll allow them over ocassionally under the watchful eye of their mother or something like that. But you do not have to invite them over ever, and you certainly don't have to watch them like you do with your nephews just because they moved in next door.

You are not wrong and you need to establish your boundaries immediately.

Three more kids is a lot (and a lot more liability with the pool). A weekend BBQ, when they are invited is one thing, but this daily thing will become much more of a habit than you will ever want, and that she will expect.

These are your BIL's GF's kids, it's not up to you to make things fair for them. Sorry if that sounds mean but it is what it is. You have every right to say who spends time in your home, yard, pool, whatever.

Your husband needs to talk to his brother and explain things. DO NOT let them take advantage of you.

No, no and NO. If she wants her kids to swim, she is to come WITH her kids and watch HER kids and LEAVE with her kids. And ONLY when invited. You did nothing wrong here. Happy that your DH backs you up. God knows the nephews are probably grateful to be able get away from the other 3 kids!

Wow, she's got some nerve asking that of you. Though honestly, it's time for BIL's kids to be home alone or with HER, instead of at your house.

Leaving stepkids out because you don't like them even though they've done nothing wrong, or because you want your own "perfect family" while pretending they don't exist, or doing it because you are just mean and evil is a no-no.

However, actions have consequences. If kids are unruly and parents allow that, then kids aren't going to be asked to be included. That's the situation here: the kids are unruly, BM isn't doing anything to correct their behavior, and so they're not allowed to be included. That is less the kids' fault and more the parent's for not correcting their bad behavior.

This is probably going to have to be a conversation that is had with your BIL, though. You get benefit from his kids coming over by them entertaining your littles. However, BIL is probably days away from his GF making him choose his family or her, so it's quite possible that he'll stop allowing his kids to visit (if he's a moron). I am assuming he is your DH's brother, so I think your DH needs to have a conversation with his brother about your family's standing on this.

IF you want to be more equitable, then you and DH could decide on rules that her kids need to follow and start with some supervised visitations/family dinners with her kids. If she can get her kids to follow those rules, then perhaps, eventually, they'll have the same privilege as BIL's kids do with coming over.

And really, this is a privilege. You all owe nothing to BIL and his kids. This isn't a situation where you're trying to blend your kids with your DH's. You could call BIL today and tell him to fly a kite and ban his kids, and there is nothing BIL could do about it. We're not even in the same ballpark as "fair and equitable treatment" with how it's discussed here because you're talking about extended family kids, not ones living under your roof who are yours or your DH's children. That level of separation means a whole lot, especially when it comes to acceptable behaviors and privileges. Like, I can walk around my house naked, but that doesn't mean my SIL can. It's MY house, and what I'm free to do and allow for myself and its inhabitants is very different than what I allow from others.

There are homeless kids on the streets you forgot to invite to your bbq. Do you feel guilty about that? Then don't feel guilty about saying no to your BIL's gf's kids who are so far removed from yours they may as well be some random person's kids. 

Your BIL is welcome to see if he and gf can blend but what does their relationship experiment have to do with your family routine and responsibilities? Surely someone other than you has been watching over the feral kids in the afternoon all these years. That person can keep on doing that. 

Those kids are of no relation to you. You are not obligated to go out if your way for them. It's one thing if it was an emergency and she asked for your help. It's something completely different when you are basically being asked to watch her kids. Plus in your defense you are working, just because you are home doesn't change that fact. 

Even if they were well behaved kids asking one person to be responsible for 7 kids is asking too much.