I think your fiance' sounds so narrow-minded, bigoted, smug, self-righteous, judgemental and unfair. Once someone has completed their term, they've done their time, they deserve a second chance. If it were murder, rape, pedophilia I could understand. But only dealing drugs? Seriously, everyone deserves a second chance, is he (and you) going to hold your sister's partner's crimes against him FOREVER??? FFS. And OF COURSE he doesn't care, because it's not his family. It's only yours, so it doesn't affect him. So he's selfish as well in not considering your feelings or your own flesh and blood - your sister's feelings. I wonder if he might be different if it were his family, or would he still be such a nasty and narrow-minded smug person?
In the country I live in (Australia) the husband of a very prominent female Labour MP served several years for heroin dealing. He paid for his crime, he came out, got a job with the government and he is doing well. It never held him back. Because people here weren't so narrow-minded, so self-righteous and such bigots. And his wife is touted as a future PM so he will one day maybe be the husband of the future PM. And he was inside for heroin trafficking.
People go to jail, pay their dues, do their time and should be given a second chance. Your future husband sounds like a real self-righteous and narrow-minded selfish arsehole. I think I'd prefer to hang around your sister's partner any day, when I consider what your fiance is like!
You will really, REALLY regret it if you don't invite him. He has been with your sister longer than your idiot fiance has been around. Tell fiance straight; that BIL deserves a second chance, and if he doesn't like it, he can call off the wedding. I'd do that anyway if I were you, his attitude is truly unbecoming in a husband imo and as I said, he sounds like a nasty judgemental and smug self-righteous bigot, looking down on people 'lesser' than him who made a mistake. Sure you want to marry someone so judgemetal? Give your BIL a second chance! He's paid his debt to society, people make mistakes. He's not a murderer or rapist ffs. BIL is your family and you will regret allowing your fiance to tear your family apart like this. I'd even be prepared to call off the wedding if fiance didn't change his attitude.