Is it normal that i'm such a concerned sister?

I am writing this post, because I am very worried about my sister and her relationship with her boyfriend. They have been on an off again for almost a year now and they seem very happy together. When I met my sisters boyfriend, he was very polite, friendly and easy to talk to. I told my sister you found a great guy don't lose him, but as their relationship grew his true colors came out. She told me that he is a pot smoker. Smokes pot everyday: in the morning before work, on his lunch, after work, while he plays video games and right before he goes to bed. She told me if he doesn't have pot then he gets real angry.

We just went on vacation to a water park this year and i witnessed his temper there with her. They had been in the lazy river just floating on inner tubes having a good time and there was this sprinkler that shot out water and he was pushing her towards there and she didn't want to get her hair wet so she jumped out of the inner tube and just said no i don't want to get wet. Well he took this the wrong way and started yelling at her. He told her that she was no fun and needed to change her attitude. So she comes and finds me and tells me that she doesn't want to be around him, so we go on a walk to talk. She's crying to me saying what did i do wrong. I told her you didn't do anything wrong he has an anger problem. She just started crying. Then he comes walking really fast towards us saying why are you walking away from me we need to talk. So we go back and I go with her because I am worried for her. He just starts yelling at the top of his lungs saying why do you always have to do this to me, you always do this blame me for stuff I didn't do. etc etc. This carried on for at least 20 minutes. Finally I tell my sister lets go swim and let him cool off. So as were getting up he gets up and grabs her arm and says your not going anywhere your staying with me so we can talk. She is crying and just trembling. Then she told me let me just talk to him by myself so I left, but still keeping her in my sight. After that everything was fine with them.

There also has been other numerous amounts of times where I have been with her out to eat or shopping and he calls her cursing at her for no reason. He even has to know where she is at all times and is always with her. My parents also do not like him at all. Is he a nice boyfriend only when he's on the pot or does he need anger management counseling or medication. Am I being too concerned for my sister? Please if anyone has any answers or suggestions I am open to hear anything.

Thank you,

Concerned Sister