As far as I know they stay with you. Although I seem to remember reading that in some HMs they move back in with their family for a while if you make them mad enough... so that could be the case in SI too. Wait a second and I'll have a look around to see if I can find anything...
Hmm... nothing yet. It would seem that the two main HM sites (Harvest Moon Forever and fogu) don't have much up on marriage in SI at all yet. Keep an eye out, you'll probably fins something sooner or later
fogu has like, a super SI section it *has* marriage
I guess you havent even looked at that section, as his questions isnt even answered there, though there is an events section, but it cant be clicked. --- Wizard's First Rule: People are stupid; given proper motivation, almost anyone will believe almost anything.