conflicted

I have a struggle of late that I don't know how to handle.  I have recently acquired a 17 year old child in my home.  My brother.  We have been playing with the idea of letting him stay with us not just for the summer but for his senior year. however, I just dont' know if that is going to work out.

Here is my dilemma, he and my husband don't really get along which make things difficult for me.  It means that Mr. S and I fight more often and always seem to just have this tension floating around.  I feel like I am always in the middle of a fight between the two of them.
And I don't know how much I can trust Little Brother.  I think he is a good kid but a few less than squeaky clean things have happened in my home in the last few weeks and while I can't pin them on anyone for sure, I do know Mr. S better and he knows what I will and will not tolerate in my home and the consequences to him for violating my trust are much more severe.  

What do I do? The scary sad part is our mom doesn't want Little Brother back. (parents are in the middle of a divorce and dad has no place for a kid) She would take him don't get me wrong but it would be grudgingly and that just makes me feel bad-especially since she has 3 other kids at home still.  I don't want him to feel like he isn't wanted, but I can't handle the constant frustration in my home. 

Will things get better when school starts for him? Do I trust him? Do I give him a really short leash?  Do I just send him home for the sake of my sanity and marriage?  I don't know how to raise a teenager, especially one who is mostly raised, I only have dogs.....................

Help?