How can I set rules for my child's friends?

There are two girls, 8 & 10, who live down the street and come over to play with my daughter (who's 4) quite a bit. They are seriously two of the sweetest girls I've ever met and they are wonderful with my kid. But it's like they have no idea how to act at someone's house. I've met their mom a couple of times and I don't think think their home life is all that stable. I don't think there's any abuse or neglect, but it's just like a mixture of wrapped up in her own stuff and cluelessness as to how to raise kids.

For example, they came over one time and went up to my daughter's bedroom to play. They were there for about 45 minutes while I vacuumed and did laundry, so I couldn't hear what was going on up there. Then they went outside to play and I went upstairs to put her laundry away and it was unlike anything I'd ever seen before. The mattress was halfway on the floor, every toy was everywhere, a rocking chair was upside down, a popcicle was melting on the carpet, a coke was laying sideways on the train table in a pool of coke, they had gotten clothes I'd stored in the guest bedroom and were playing dress up and they were all over the floor, a curtain was pulled down. I don't even know how they got it like this in 45 minutes.

Another time they decided to have a picnic and took a bunch of food out of the refrigerator, took it outside ate a little bit of it, then left it on the deck. I came out to find an entire package of cookies, with only like 3 eaten, covered in ants. A whole package of lunch meat, OJ, the works.

When they have done these things, I've calmly explained why they aren't allowed and they just say sorry, help me clean them up, and they really don't do them anymore. Occasionally they'll forget and slip up, but they really do try. But then just when I think they have a grasp on it, they will do something else that I've never told them not to do, like put wet bathing suits on my antique dresser. I can't watch them every second because I have another child.

I can't really talk to their mother about this because for one thing, she's likely the problem, but also because I think she would just punish them rather than teach them what is appropriate. I can't imagine, at the age of 10, going to someone's house and going through someone's closet or refrigerator without asking. I want them to keep coming over because they are so sweet to my little girl and they really do have good intentions. They will surprise me with poems they wrote for me or offer to do my dishes. I'm just not sure how I should go about teaching them how to behave without them destroying my house or me going insane.

Also, they have a cousin who comes over sometimes and she's just as much of a handful, but unlike them, she doesn't listen to rules. I have told her that if she doesn't follow the rules she has to go home, but it hasn't gotten that far yet. Unfortunately, if she goes home, the other girls have to leave too because she's staying at their house. Is it fair to send them all home if only she is misbehaving?

What do you think I should do?

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