You should not try to reason with her or talk to her because that will just make her more mad or upset if you make her a present or apologize she will probably forgive you. Of you did not do anything wrong than give her some space. Also if it's a younger sister give her a present if it's an older sister use my other advice :)
You tell your boyfriend you need to talk to his sister and talk to her and work it out if that doesn't work ignore her and start paying more attention to your boyfriend.
Make an effort to learn more about her. Show interest in her taste of music, her friends, tv shows, etc. Act like more of a friend than a sister.
madara doesnt have a sister only a brother named izuno or somthing like that.
do it behind her back
he doesnt have a sister -_-
she doesnt have a sister she has a brother Cameron
no. she doesnt
she doesnt have a sister she has a younger brother Blaize Pearman
A lot of the times yes, but it doesnt really mean anything.
no she doesnt have a sister
she doesnt have a sister she has a younger brother Blaize Pearman
No she doesnt she has a step sister!
he doesnt have a sister, only an older brother paul
She doesnt even have a sister