STORY: ‘I’m Addicted to séx with my fiancée’s sister and now she’s suggesting we all have a threésome together

I’m having séx with my fiancée’s sister and now she’s suggesting we all have a threésome together.
I’m 28 and I’ve been with my fiancée for five years. She is 30 and we’re getting married next year.
We have bought a flat together and I thought nothing could rock my world but then her 22-year-old sister fell
on hard times and moved in with us, along with her five-year-old son. My fiancée insisted she could stay until she got back on her feet but it’s been six months now.
She admitted to me that she’d been engaging in próstitution to feed her habit – and said she would have séx with me for free if I kept my mouth shut.
I couldn’t resist and I’ve been having regular séx with her. I don’t have feelings for her but she’s much wilder than my fiancée.
She suggested we could have an amazing session with my fiancée. I just laughed it off at first but she told me they’ve done it twice in the past when my fiancée was drunk.
My fiancée is 31 next month so my lover’s plan is to get her totally drunk so we can get her into bed.  It’s wrong but I can’t get the whole thing out of my mind. Please what can i do?
Friends please share your thoughts!!