AM I WRONG?? PLEASE HELP !! I've been in a relationship for the past 3 years... ?

my boyfriend is an incredibly jealous person, and this have caused us to break up over time, we always seem to work things out and get back together. My sister, has always seemed to get alone with him, she now sick, has been for a few months now, nothing terminal or mental. Well, i don't know which of the two to blame, cuz my sister is always playing the victim and acting even more sick when my boyfriend is around, and she's no little girl or teenager any more, so my boyfriend hugs her, kisses her on the cheek, buys her food, calls her up etc.... Its like he worries to much about her, but its not necessary at all ! I don't know who's right or wrong anymore... and he does this even more when we're upset. Seems like she takes advantage of him, and he just follows her lead! I don't know what to do, i get mad, and i don't know if jealousy also has to do with it !! We're speakin bout adults here ! Who's right, who's wrong?