My sister-in-law seduced me

I was just 27 and working close to my sister-in-law workplace and we used to have lunch together. Anyway, my brother works week on week off and was away a lot, my sister-in-law asked me to move in with them. They've a 4-year-old son, who is in day care while works. A cheaper place to live suited me greatly, while my brother was away one week, my sister-in-law joined me in my bed and seduced me. I'm not active on the dating scene and my sister-in-law is bi-sexual and we have regular sex when my brother is away.