I think my boyyfriend is cheating on me with my step sister part 1?

Ok so I've been with my boyfriend for 5 years now its never been a strong relationship and he has cheated a few times before. So I wouldn't be surprised if he were doing it again.. the problem I'm having is who I think he is cheating with. Me and my step sister have known each other almost our how life we are just alike we even look alike weird I know. And I know that she has messed around with another one of our sisters ex es before so I know she is capable of doing it again. Now like I said we are very close and my boyfriend is always with me so it's usually the three of us and lately I've been getting this gut feeling that they are messing around behind my back.

I think my boyyfriend is cheating on me with my step sister part 1?
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