How do you access the Twins?
It's a random event inside the corner shop during the afternoon that will trigger that SB. You also need a bit of depravity (DL 15) :D. Then you should be able to get to their house when you walk about outside. As a side note, that side story is not done yet, I hope to continue that one when I am bit more happy with the content inside the house. The twins story is about half way done right now.