I've married her 3 times and the key to a happy marriage is to eliminate that creep before you get married.
The way I do it is as follows:
-have her follow, take Aerins key and have her wait out of sight of his front door around 3 a.m.
-sneak in to his house and to his bedside while he's sleeping (he must be asleep in bed), kill him quietly with a single arrow or dagger then shut his bedroom door on the way out (must be shut, otherwise I believe she will see as this didn't work when killing him elsewhere in his home)
-go back to Mjoll, tell her to part ways then immediately propose to her (very romantic)
-setup the wedding then wait til the appropriate time for the ceremony and she should be waiting for you at the altar, blissfuly unaware of Aerins whereabouts.
-you will now be happily married to Mjoll without worry about her creepy stalker.