I muck around with my partners brother

It all started 6 months ago it started off as some fun i mean we were both going through a rough patch in out realtionships and one thing lead to another, i have always been attracted to him they are almost the same person, same personailty and almost look alike! except he knows how to treat a girl better then my partner, i really want to stop but i love the attention please help!!! btw we have never engadged in anything more then a kiss and some touching!!