If the girl in question likes you then you won't need to convince her to let you be her boyfriend, you must wait for the signs to see if you think she is interested, let the girl make the move, don't be pushy, be patient.
don't try and convince her, just let her choose, because really its her decision and its really low to try and talk down about her bf and she might take it personal
You prove it to him. You don't just say it, do it.
Smile a lot and hold his hand. He should get the message.
Then she has to choose you or her boyfriend.
Does the girl know you like her? How long have you an and your boyfriend been together? And are you asking what should you do?
Wanting to love a girl who already has a boyfriend,and trying to convince her that he can love her more than him.
Try an show her how cool you are compared to her boyfriend
If he is her boyfriend, hopefully, she would like him.
Wait till she doesn't have a boyfriend or move on
Treat her better than her boyfriend does.
convince them it is just dyed clothing
First show your brother all nice things your boyfriend is and if he still doesn't like him tell you brother to talk to your boyfriend so your brother can get to know him. And he should like you boyfriend after he gets to know him