Have just come in on this one. What a difficulty! But I think that you have really identified the problem yourself i.e. that your SIL just lets him get away with anything and doesn't seem to see that he is totally indulged. If you are in any doubt don't do it!!
Until she sorts it out herself I agree with the previous suggestion that your OH goes over there. I wouldn't have them with you or pay for anything - it's just opening yourselves up to being used. Hard though that may seem.