I think it is time for you and your husband to have a heart to heart talk.......tell him what she is doing to you not on finacially, but emotionally.........and if she can buy an Xbox, she can pay you..........ask him what he would do if he was living with you and your husband.......and you were his sister..........would he "cheat" his family as she is doing to you? Tell him that she is being seflish and unfair.......as well as taking advantage of the offer you have made.........and find out why he doesn't see it that way...........you should also tell him that this is going to cause family issues down the raod because you are losing all respect for his sister and you are going to get angry.......
Then you need to either make your SIL stay at home for a night, or tell her in a letter, left in her room, that this has to stop. You don't have to say that you will kick her out, but you might hint at it.........stating that you feel you are being taken advantage of, that she agreed on certain things, and she is not upholding her end of the bargain.......that if she is going to eat at the house, every meal is 5.00........which you will start tracking.......and then, I would make up a "contract" and make sure she either signs it or moves.........with all the money issues stated up front and include meals, etc...........make it like a lease if you must, but get your point across...........then you can sue her if you need to......when she pays you, give her a receipt.......and keep a log.....of meals, payments etc............start treating her like a person you rented the room too............
If your husband doesn't understand or jump on board, then you still need to do it.........tell him too.........that you would expect this from someone in your family to honor their word and that you expect it from his family too..........yes, there might be times when she has a legit excuse, but right now that doesn't seem to be the case...........you might even let her make up the differrence by babysitting, if you can trust her to do that.......but either way, she needs to become an adult.....no matter how old she is....
Good Luck and I hope this doesn't damage your relationship with your husband (lose respect for him) and the rest of his family too..........is there anyone else in the family you can talk to that might be able to explain this to her and get her on the right page?