You should also know that you may relocate your family to any one of the three house locations contained in Hearthfire. I told Mjoll to travel to my Falkreath house, and I promptly fast traveled there myself.
Now, I have tried to rid myself of Aerin in many ways, I've shouted him off cliffs, I've stuck him in locked rooms, I've cast frenzy on him, I've told Mjoll to attack him, I've told my housecarls to kill him, I've told the Brotherhood he needs to go. All of the above have always resulted in Mjoll ceasing all conversation and returning to Riften.
Now at my house in Falkreath, I begin to slowly walk to the door, laden with all sorts of IRON. I arrive and guess who runs outside to tell me that "Mjoll's never been this upset..."? Yep, that guy.
I store my iron and glass, and start to see what all he does. Typically, in my Windhelm house, Aerin would vacate the premises within thirty seconds after my arrival. In Hearthfire this is not the case. He sprints in Mjoll's footsteps around my house, knocking things over, and even going as far as staring intently at Mjoll whilst she sleeps. Naturally this upset me, and I began looking for ways to rid myself of Aerin once again.
After many attempts, all failing, many loads, and many saves, I became frustrated, not being able to find a way forward. In my anger, I ordered Mjoll to take her lousy pet back to Windhelm. Being upset, and not having enough IRON to make anything else, I followed them outside.
This was when Skyrim improved immensely for me. Mjoll instead of disappearing and reappearing at Windhelm decides to walk. Yes, that's right. Walk to Windhelm. Naturally, Aerin must follow. The two were no more than 100 feet outside my house when the event happened.
I saw a red blip appear on my compass, Mjoll draws her enchanted ebony sword. Aerin draws a dagger. Out of the woods of Falkreath emerged a Vampire Nightlord. Aerin was swiftly consumed and reanimated by the drain life reanimate combo. Mjoll was forced to strike him down herself. After watching the whole exchange, and fearing Mjoll may find me responsible, I quickly fast traveled to Windhelm. I ran into my house, frantically searched for Mjoll, and upon not finding her, decided to wait for 12 hours.
After waiting, I saw Mjoll sitting in a chair downstairs. Aerin is an ash pile in the woods. I approach Mjoll, nervous of the outcome, already prepping my rage engines for firing. And then, conversation! Mjoll was apparently ok with the fact that Aerin was dead, I assume it was because a Vampire from Dawnguard sealed the deal.
Regardless, I credit this outcome to both DLCs from Bethesda, thank you for ridding me of the single most annoying character in the game. He was even more annoying than the Adoring Fan.
Thanks for reading my story, I love Skyrim.