I think that the Count was having an affair, but his wife found out and was going to besmirch his good name, so he hired the Dark Brotherhood to kill her. However, they used this killing as leverage to manipulate the Count into giving them a base in Cheydinhal, as well as money and resources.
This, pretty much, sounds reasonable.
I made a quest that's about her in Kragenir's Death Quest. Takes a little creative spin on the rumor, but the count and the DB are involved. And stairs.