no
LOL AT YOU, why do that?.
if they are your best friend they should understand
just tell him and get over it you should feel happy for her she is your best friend
you should try asking your boyfriend to talk to him
Talk to her about it.
I think you should ditch your best friend and replace her with another person who won't betray your trust by dating your boyfriend. That's what you should do.
You're jealous, so back to her.
Hit him
Yes u should tell him . Before some one elice does or he mite not be your friend anymore.
Ask them to be your boyfriend/girlfriend (i dunno which gender you are so...)
You should tell her, they should be ok with it because they are not in love with their cousin and plus if she is really your best friend it will be cool with her.
well if you like your best friend then you should go out with him|her... but if you don't your best friend then you should tell him|her, that you are going out with someone already.