I think (well it is what I did), talk to the groom and agree to kill the old guy. Go upstairs and grass him up.
Go down again and the bugger has fled. Talk to the bride, find the house, find the cellar, rain on his parade (so you can see them). Kill them.
Something like that anyway.
My approach to RPG games is like my approach to life. Stumble around and see what happens. You can always come back and kill people later if you want even if you can't remember why.
Rather bizarrely in computer games I always seem to be over-leveled. This doesn't happen so much irl.
I still don't know if Magisters are goodies or baddies. It concerns me slightly as I should have an opinion one way or the other by now (4th playthrough) but I don't. Such is life.