Because it breaks a certain trust barrier. You bring your friend close to your family, the females of your family and he uses your trust, your friendship as a tool to score your sister. That’s not cool.
There is a chance of relationship not working out. Which will hamper everyone.
Imagining your friends doing stuff to your sister is just eewww.
And you know how sick and perverted your friends can be, you know every little detail, all their bad habits because how close you are to each other. These tiny details that you know hampers your judgement.
Would also like to add that some people talk shit about the girl after break up, or some men flex their private life with their gf among friends. You don’t want that to happen to your sister especially among own friend circle.