My ex's bestfriend (they're so close they call each other brothers) has started to flirt with me and has asked me what I would do if he kissed me. I don't like him like that, although he is one of my good friends.

On top of this my ex is trying to get back with me. I still like him, but I have trust issues because of relationships I've been exposed to my entire life (every relationship ive seen has ended due to cheating). What do I do?