How would you feel if your best friend and your younger brother started having feelings for one another?

ok, so my best friend told her brother(who is only a year and a half younger than me) that i needed a date for prom, so he asked me. while we were there we had a great time! it was definitely the best dance i'd ever been to! i don't normally have fun at dances and he had a good time too. he kept making comments like " you know if we were dating mandy might be mad at me bc ur her best friend and all, but she's mad at me a lot anyway so i guess it doesn't matter." i asked him where that came from and he was like "i don't know" but we really had a great time together and he asked me if i would come to one of his baseball games and said that we could go bowling later and maybe ice-skating. i really would like to go out with him again and maybe go steady because he's such a great guy. the only problem is i wonder how my best friend would feel. so how would you feel in that situation?