I *love* marriage law fics... but you've got a few plot holes going. I think it's a great start, to be sure, and I'm really glad there's one post-hbp so that Snape's blood is taken into account... however, I'd really like to see you address how Sirius is a.) not dead and b.) (being that he is an escaped convict) would be an acceptable match. Wouldn't the ministry just arrest him right after they got married? Also... Tonks is a half-blood. What will they do with her?
Please, please keep writing. Like I said... I love marriage law fics and I really like the idea to do one with Sirius. Fill in the holes and this could be absolutely superb.