Wtf!? lol i need sum **** explained please?

ok im 15 my bf is a twin his twin brother is dating my sister, his name is aaron and my bf is zack my sister is kayla and my name is alli. ok well me n zack have been dating for almost a year and kayla and aaron have been dating a little longer.. well appereantly everone whats too change aronud i guess? i was hanging out with aaron yesterday and we was smokin gettin high and he kissed me and told me he whanted to be with me then i got told tht my sister had sex with my boyfriend, What the hell is going on?? im confused? cuz aarons a good guy, i thought zack was guess not, and my sister is goin along with this? when they started dating i was mad at her cuz i liked aaron alot but she dated him, i guess i still like him maybe? but zack told me he loved me.. idk can anyone explain this to were i can understand it?