I caught my sister in bed with my boyfriend while her boyfriend was downstairs?

well. basically. I CAUGHT MY BOYFRIEND IN BED WITH MY SISTER WHILE HER BOYFRIEND WAS DOWNSTAIRS!!! well I’ve been with him now for nearly 20 months, i thought he loved me, and i adore him! well DID. I’m confused! so bad, i stayed at a friends house on wednesday night and was textin him, he was comin round to mine today (Thursday) so i told him to go in the house (there was people in) and i wouldn’t be long, i went home and my sister (age 15!!!!!!)s boyfriend of 2 months (age 18) was in the living room, so i said 'oh hey Jamie wheres everyone?' he said 'Randy (my boyfriend) has just gone upstairs and Kim (my sister) has just gone to the loo' so i told him okay n everything went upstairs, walked into my sisters bedroom and they were in bed! MY 15 YEAR OLD SISTER AND MY 19 YEAR OLD BOYFRIEND!!! i flipped straight away, i was like OMG GET OFF OF HIM YOU ***HOLE!!! (she was on top of him and was about to take off his shirt) and i ran downsairs crying, her boyfriend was like 'Sabrina you ok? whats happened' and gave me a hug. i was like 'no im not really ok! do you know what your little tart of a girlfriend and my ***hole of a boyfriend have bee doin up there!?!?!!' i was proper mad, he was just like no why i said well go upstairs then! so he ran upstairs to see what was going on and world war 3 kicked off! they were all arguein,, her boyfriend hit my boyfriend, everyone started yellin at each other, i just sat out on the drive way crying, my boyfriend came out to me and put his arm round me, i threw him away and told him to f*** offf out of my life forever, and my sisters boyfriend ran out of the house and shouted 'its over Kim! get over yourself *****!' he came over to me and asked me if i was ok, sat with his arm round me talkin to me for a while and then he went home, my boyfriend wasnt tryin to make excuses or anything he was just like 'she offered it ona plate, would you say no? im really sorry, i love you proper millions, you dont know how much i regret this babe im so sorry i really love you, have i messed everything up?' I just didn’t know what to do!!! luckily my parents are at the movies when this happened so they missed it all, but my sister managed to wreck two relationships in the space of 10 minutes. im so confused! can anyone help me please?! should i take my boyfriend back?! should i finish him forever?! i really love him! im so confused! i just wish i could wake up & this was all a bad dream, please help me and should i tell my parents that this happened as well???