Do I tell my cousin she's being cheated on?

so I have this really stupid cousin on my moms side and she has been dating the same boy for like a year or more and he cheats on her all the time do I tell.. her even if I did she wouldnt care or believe me what should I do?

u love your because rgt.. honey it wudnt hur just to try and tok too her.. list suspicious things that he does dare her too prove you are wrong that he is nt cheating.. all the best

you tell her