Sister melodrama was out of place
Maybe it worked a bit to show how he was ruining his life and everybody else's.
But the jealousy, possesiveness, killing his best friend over nothing, her demise...it was all clumsy, unnecessary and over the top. It almost felt like they wanted to add minutes with a b story filler, and just threw in a "what if he has a little sister and he's jealous". What does that do to the story, other than make it creepy?
It could have worked if he tried to keep her clean but Manny was the corruptive force. Here it's the opposite, so Tony looks stupid or incestuous for getting angry. Also her overacting was annoying.