what are the legal consequences of marrying an

illegal alien so that they can get their greencard?

Assuming you're a US citizen, then there's no consequence that I'm aware of. As long as your marriage is (provably) for love and not for the express purpose of getting a green card, I don't think the sponsoring US citizen has violated any rules.USCIS will probably interview both of you, ask for lot of documentation and do a thorough job in verify ing that the marriage is genuine. If they suspect the marriage is for GC only, they will probably not allow the GC to be given. If they can prove it, then you'll very likely BOTH be in trouble with the law. But if it's a genuine marriage, and you live together all the time, have lots of joint finances, file taxes together, maybe have kids etc, then I think that's not a worry at all.I'm not a lawyer; I'm only a slightly informed person attempting to add some info.

going to jail...