Kim,
The way I see it, you have two separate issues at hand:
1) Your husband said yes to his brother without talking to you about it first. Not to mention that you were already generous to his sister and got stabbed in the back for it. That wasn't very respectful of you for him to do it and I think it should be addressed.
2) You shouldn't have to open up your home to anyone if you don't want to. And, you have every right to say "No".
My younger brother lived with us for a year. He was 24 and trying to save to pay for a wedding and buy a place of his own with his fiance. He could have rented, but felt it was such a waste. He wanted to leave home because his dad (my step-dad) got remarried to an absolute b*tch. Seriously. When he asked me if it was ok, I told him I had to talk to my husband about it. We came up with some ground rules and it all worked out great. Actually, I feel it brought me closer to my baby brother and I kinda missed him when he left.
I think there are ways to help people without having them invade your family-life. I'm not sure how old your brother-in-law is or what the situation is or why he hasn't been working...but maybe he should have to take some responsibility on his own instead of turning to your husband.
I'm all for being there for family and helping out when I can but there comes a point when it's time to say "enough is enough".
I think you should talk to your husband and just tell him that you are not comfortable with the idea of anyone else living with you at this point. It was a train-wreck before and you don't want to take that chance again.
Then, I think you should discuss why he felt it was ok to say yes without talking to you about it.
Good luck and keep us posted.
T.