Ya know, I'm not one to give out advice (which is why I'm sitting here at work typing this out to you!) but this is really a crappy way to start out a marriage.
And as long as I'm not giving out advice which you never asked me for personally, you really need to talk with your soon to be husband and let him know that you two are not responsible for the uncle. I sympathize with him being homeless for 10 years, but why all of a sudden do you have to deal with it? It's hard enough living with a spouse and trying to get to know them, much less housing every family member that comes along. Don't know if it's too late to do so, but you really need to NOT take in this uncle. He's not your responsibility, he's not your FFIL's responsibility. And if your husband doesn't put his foot down, you may have to do so, which could cause resentment toward him on your part.
Okay, enough of the free advice. . . just not a good way to start out a marriage. Who knows - this is probably the first of FFIL's manipulative acts. If he "gets away" with this, then I'm sure he'll feel like he can dump anything on the both of you. Tell the future hubby to get a stiffer backbone (and I hope it doesn't sound like I'm calling him a name, but this kinda confrontation is hard to do sometimes) and stand up to the old man and adopt Nancy Reagan's slogan: JUST SAY NO!
