Trashed Trailer

I have not been the best blogger lately. For some reason, sitting down and writing about all the things that have been upsetting me lately, has just seemed too daunting.
For the past two weekends, Greg's sister, her fiance, and their kids (and kid's friends) have come up to spend the weekend at the campsite. I really like Greg's sister, but I think that she is thoughtless, she let's her kids run wild, she stays in my trailer and does not clean up after herself, or her kids, she lets me do all the cooking and cleaning while she sits and does jigsaw puzzles and puts her cigarette butts out on my patio. Her kids will walk right past me in the morning without even a hello, and when I say good morning to them, they don't even acknowledge me. The kids will lie backwards on the couch with their feet up against the window, and don't lift a finger to clean up after themselves. I was stupid enough to do their lunch dishes because I couldn't look at them anymore. (They are 13) She provides food, but that's it. I have spent the past two weekends pissed off. This past weekend, I did not know that they were coming up, it was kind of thrown at me at the last minute. It has taken me DAYS to calm down, and decide how I was going to approach Greg about it. Basically, I told him that if they were going to continue to come up, a) I needed to know about it in advance, b) that his sister was to clean up after herself and her kids on an ongoing basis, and not just at the end of the weekend, c) that they were to respect our belongings, d) that I am not going to entertain, or babysit her kids. I absolutely LOVE going to the trailer, but I have had some serious thoughts lately about setting it on fire. His mom is up there every weekend, she has her own place. As it is, she never leaves us alone. She is ALWAYS at our site. I love his mom, but honestly! Give me a break! Let's just say, that Greg and I never have a romantic moment on weekends. At dinner time, his grandparents always seem to know when the supper bell is ringing. I don't want to sound like a bitch, and I realize that when you marry someone, you marry their family, but how can I be diplomatic about this and keep my sanity???? Greg seems to be understanding about how I feel, but I really think that HE needs to say something to them. Am I wrong?
Anyway, I thought I had calmed down about this, but my blood has started boiling again.
I will take any, and all, suggestions.