Not the best situation to be in. I half agree with ilovemyglo, but I can understand so close to the wedding that those thoughts can be a little difficult. I think someone should sit down with FFIL & FBIL and in no uncertain terms tell them what's what.
Are ya'll the only family left after the two "losers"? If so - have you looked into an assistance program? Maybe they have a halfway house or something that could be of more assistance to the Uncle than ya'll could be. With all the health problems you have suggested maybe even an assisted living facility? He probably qualifies under medicaid or medicare as you have told us that he was homeless meaning he has no assets.