why give me suspicions BABY?!

everything seems fine in our relationship.. normal arguments time to time which keeps our relationship healthy "i like to think".. 

i had a suspicion of my brother in law cheating on my sister.. so i asked my fionce and he kicked off really bad. so i told my sister so she could deal with it.. anyway my brother in law shortly called asking why I assumed such a thing and why i told his wife. any way i explained and i said im her sister obviously ill tell her.. and he said listen ur my sister in lqw im not cheating on ir sister but you should know as a brother in law, your fionce is cheating on you.. think about that before you marry him.. by the way my fionce and brother in law work together.. they always seem to go to the gym/ football together too.. 

ps. im comfused is he just saying that cz hes pissed off ive told his wife or is it actually true? 

i mean why would we be planning a wedding spend nearly 8,000 on a marrisge if he was cheating?!