Is this cheating and how do I let her know

I’m usually not that good talking about things because when I was growing up I was always told to not say anything. This been happening for a few years, one of my family members boyfriends has been cheating on her for about 5 years as she has a 4 year old son and having another baby in November. My family member has told us about her bf cheating and showed us photos of the girls he cheated on her with. Some people have warned us to stay away from him. The week after I turned 18 he tried to dm me and ask for my content, I told one of the family members about it but they told me not to tell her, now they’re getting married and want me to be a bridesmaid in the wedding. It doesn’t help that she’s in denial about him cheating.

I feel like the right thing to do is to let her know except she wouldn’t listen to me the first time when I told her that her bf was treating her badly.

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