<human>: Give me the gist of the text in a nutshell

My husband and I have been together for 5 years, married for one. We won't be having any kids for at least five years. I met his sister when we first started dating. 
They have the same features and personalities and everything. 
My husband (Mark) knows how much his sister (Jan) looks like him. 
Whenever I'm around her, I get the same feelings for her that I have for my husband and I'm so confused. 
She flirts quite heavily with me and I flirt back a bit. My husband doesn't mind at all, which makes things worse. I've told him to not leave us alone because I feel like Jan escalates with flirting when mark isn't there, but he just laughs and says things like "So..?" 
He obviously trusts me and that's great, but I'm scared. 
Jan's staying in our state while taking care of their grandfather, so she'll be around for a while. She's also toying with the idea of buying a house a few doors down from us and I'm going crazy. 
Jan's newly single and talks about how lonely she is a lot. I never know what to say back to her. I just say "You'll find someone" and hold hands with Mark. 
I'm so disappointed in myself. I'm failing myself and my husband. 

Does anyone know how to fix this?
<bot>: Having feelings for my flirty sister in-law because she's basically the same person as my husband. What do?