How to tell it to his best friend's sister?

Incest & Cuckold: After two months dating her brother's best friend he started to loose interest in her. Everything seemed to be great with them but there was something the guy wasn't telling her. Finally he got the courage to tell her that he and her brother shared their girlfriends before he started dating his best friend's sister, they missed it a lot but didn't want to tell her. At the end there wasn't any problem, she only said if her brother was ok with it she was ok with too.