First of all, with all due respect, I think you should throw the letter out the the window and quit having conversations with your sister through your mom. I believe much more could and would be accomplished if you'd just go over to their house, sit down and have dinner with them and talk it out face to face. A little "face time" will accomplish much more than any letter will and it will DEFINITELY accomplish more than talking it out "through the grapevine" (ie., your mother). Forgive me if that's a little blunt, but my younger sisters do the same thing and it really bothers me. If they have a problem with something I do, instead of talking to me about it they tell my mom (keep in mind that we're all adults...not kids).
Second, it really is their choice since it's their home. If you can't come to some sort of an agreement then YOU have a choice...either abide by their rules or stay somewhere else.
I really think you should go and talk to them in person though.
