Yeah.. 13 is a little young to make any assumptions about them knowing rules. Spell it out for her.
1) While we are gone, no company is allowed over with you. Your mom or brother can come with you if you are uncomfortable for some reason (dark, stormy weather, etc), but no one else. Then follow up with an explanation, since she likely has no clue about anything beyond a 13 year olds' world: One, as a matter of privacy, I don't like strangers in my home, especially around my dogs; and two there are liability issues that you may not be old enough to understand, but if something were to happen to someone or they were to be bitten by one of the dogs--even accidentally while playing with them--there could be serious ramifications that you may not be aware of and not in control over (the other parents suing and demanding your dog be put down, etc).
2) Here is what the dogs should be fed: ____. Treats are okay, but that means only ___ much per day. Otherwise, it is bad for their health, just like if we were to eat an entire package of Oreos, instead of just 2, and the dogs can have digestive issues from it.
3) Only play with the balls outside. They mark up the walls when they bounce off, and there is always the potential for something to get knocked over and broken accidentally. If you want to play with the dogs inside or weather doesn't permit a lot of outside play, here is what you can do inside: ______ (and list them. For us, we let our pet sitter hide liver treats and tell our dog to go find it. She LOVES this game, and the treat is very small. Or you could suggest grooming them with a brush or something for indoors, or whatever).
Etc.
Whatever expectations you have, spell them out. Our dog sitter is a teen neighbor, but an older teen -- 16. I still leave a printed out list of expectations and emergency information (dogs vet info, their phone number, her weight, age,---along with info to contact us if necessary. etc)
We allow our pet sitter to stick around and watch a movie or some TV or video games, if he wants, to allow our dog to have some time with someone in the house, not just the daily potty breaks and feedings and such. But he knows the swimming pool is off limits. He swims with our kids when we are home and it isn't a big deal. But without us here, he knows it is off limits. No one swims alone at our house.
He knows where the leash is to take her on walks, and where (and what) her favorite toys are (mostly the frisbee). Everything is left on the kitchen island for ease of finding it, along with her medication and dosage schedule. It is ALL in writing. Even though he has sat for us many times. We always have it in writing. And phone contact is always ok, too. He sends us pictures of her sometimes, too. :)
But a 13 year old? You are not crazy to not want her to be bringing friends over when she is dog sitting for you. But you need to spell it out for her.