My sons father cheated on me with my little sister?

Has anyone went through anything like this?
everyone in my family noticed my 16 year old sister would stare at my boyfriend a lot, I've had family come to me concerned saying she looks at him weird. Of course I noticed but I felt I trusted him & I wasn't worried about my 16 year old sister having a crush on my man, I mean he's 21 & she's 16. She would come over to visit all the time & it's like she would purposely try to be close to him, sitting by him, standing by him, or going & sitting out on the deck or living room when he was. Then whenever we would get into arguments or fights & she was there she would offer to help him by driving him to his families home.
I kept finding condoms & the empty wrappers, I was so confused & didn't realise until way later... I remember we were cleaning the guest room together & I found one behind the bed & he said it might have been her bringing guys over but she never ever brought guys over to our house. Every time I would go somewhere I would come home to her visiting.
Then one night we got into an argument & I went to try & sleep downstairs, it was like 3 am & I hear the damn couches creaking very quietly so I went upstairs quietly & my sons father had her bent over so I beat her up & dragged her by the hair & took her home & told her mom what she had done & they were so mad at me, I threw out all his shit in the yard and threw his shoes in the bush & busted glass on him, ripped out a bunch of his hair now I'm the crazy, psycho cunt.
My sons father cheated on me with my little sister?
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