Setting some healthy boundaries?

Ok, I'll try to be brief about this (but I know I won't be!)...

Its summer, I have a 9 yr old son. We have 2 children that live in the house next door, a girl 10 and a boy 8. They play with my son most everyday. The boy is pretty sweet, quiet and generally well behaved as any 8 yr old. The girl is NOT. She is angry, mean, bossy, yells and screams at both boys a lot and generally causes a lot of problems. For a while I was pretty hard on my son because I was under the impression he was causing (or greatly contributing to) these problems. But I have tried to stand back and observe their behavior for a while and its pretty clear the girl is just not very nice.

This leads to many problems. First off, I don't particularly like someone treating my son like this and he really, really doesn't want to play with her. But because she is the boy's sister, its sort of a package deal. The parents are EXTREMELY neglectful of these kids, they are put outside early in the morning and basically aren't allowed back in all day. That is, on the days there are actually adults at home. Most days they are both alone or they just leave the 8 yr old alone outside for like 8-10 hours at a time (no phone, locked out of the house ALONE). I live in a small town. The dad is a cop and they believe this behavior is perfectly acceptable and are completely unwilling to even talk reasonably about the daughter's behavior and do not give one lick about leaving the kids (one or both) alone like this.

Ok, we have a nice, large yard with a fence, inside the fence we have a swing set, pool and trampoline. The kids (of course) want access to these things. So far, I will allow limited access to the swing and occasionally the trampoline, but I have zero intentions of letting them in the pool. Neither of them know how swim, no parents at home, blah blah. I don't want the responsibility of dealing with them and these "dangers". I have specifically told them they will not be allowed to swim under any circumstances, yet they ask CONSTANTLY. We've only been in the pool once so far and they stood on the other side of the fence staring until my son felt pressured and got out to go play with them.

So this is annoying. But it gets more annoying.

This morning my son and boy were playing with cars out front and the boy cut his fingers pretty badly on some glass. They come to me, since I'm closest (or at least that's what I assumed at first). I clean him up the best I can but he's still bleeding and I feel he needs a parents attention. I wrap his hand and take him home. About 2 mins later, he comes back with the sister and they say they cannot wake the mom up because they will get "in trouble" (which this means grounded). So I bring him back in, clean the wounds the best I am comfortable with (if it were my son I wouldn't have been so nervous, but you know your kid best, right??) and bandaged everything up.

So basically I am furious at these ****** *** neglectful parents. I mean, seriously, your kids cannot wake you up when they are BLEEDING? I had a bad feeling that I would end up being the de facto babysitter this summer and it looks like I have ended up that way. I am not cool with this, yet I am not willing to leave a kid outside bleeding out on my yard.

The parents DO NOT CARE what I say. They will not change this behavior. And before everyone tells me to call child services know some things about where I live: (1) he's a cop, so his word will trump mine any day (2) because the kids aren't physically abused and are being fed, child services will not investigate anything (3) in MS there is literally no law about the age a kid can be alone. The basic rule is that you cannot leave a kid under 5 babysitting younger children, but you can essentially leave any kid alone for however long you want.

I am so frustrated about all of this though. I don't know how to deal with it.