Best mate+ Sister this is my rant

[gbright]

Well people this is my first rant but ive never had anything worthy of a rant before so here we go
My best mate/ex best mate now who shall remain nameless has been my friend for about 2 years or so recently he got kicked out of home so he was offered to stay at my place (i still live at home) till we moved out under the proviso that he was to stay clear away from my 16 yo sister i am 22 so is he here is where it gets sticky after about a month im getting a bit suss so i confront him, he denies everything saying along the lines that he would never betray my trust like that especialy seeing as we had taken him in to our home another month goes by im still suss so one day i happen to walk past his phone witch he has left so i think stuff it n have a bit of a peek neway's found some very incriminating msg's kicked him out the next day telling him if he came near again ra ra ra all that jazz. the pearentals then got involved and now the situation is as stands
*they are still allowed to date
*he's allowed back in the house (and i have to be nice to him so as a result whenever he is here i just leave)
*me and him are not friends
thats a basic rundown NOW here's the kicker the boyfriend now makes my sister feel bad/feel like it's her fault that he never see's his friends because they hang out with me and not him. but at the same time i am being made out to be a big bad monster for the way i reacted by two of my closest female friends and it affecting our friendship in short about 4 different peoples lives have been thrown into havoc and freindships under strain because one person put a lay over his friends