Brother doesn't let his friend date his little sister?

same problem with my sister m 21 and my sis has just turned 16 i was walking to the shop and i see my mate drive past with her in the car s i ring him there and then askng wtf r u doin and he says he see her walking home and gave her a lift so i thought nothing of it, next week im in my local having a beer with the same mate who i have know for 11 years when he says to me hes in love with my sister i found that quite peodoflic so i grabed hes nuts with one hand pulled out my knife with the other and said if i see him so much as speak to my sister he wll never look at a women the same way again with that he pissed himself n front of everyone i havnt seen him since sometimes i regret it but most of the time i know it was for the best cause now she is going out with someone her own age who is a cool kid but he still got the same warning.
ppl say im obsessive, but she is so gullibly and naive, that she will get pregnant by some muppet and throw her life away everyone needs someone to watch over them