Family Issues: WWYD

First some background...

I have an older brother.
He's 28 yrs old.
He's been married once before.
Has a little girl.
Recently divorced.

While married cheated on his wife a number of times.
While divorced cheated on his girlfriends plenty of times.
In my eyes, he's lower than scum.

*We've never really gotten along and all the crap he pulls makes me dislike him more. He's also mom's FAVORITE son/child, so bonus points for him.

He is supposedly "dating" a girl. Said girl is 18 YEARS OLD. He swears he wants to get married. He lied to this girl and told her he's never been married before. He said his daughter was born outside of marriage. When he's at home with my brothers he brags about how he can nail her if he so wishes.

I think it's a HUGE INJUSTICE to allow this girl to marry him without first telling her the truth about his previous marriage and what a piece of shit he is.

Mom has meddled in everybody's freaking business before (in regards to relationships) when the person wasn't to her liking but is going through with this major bullshit. She claims to want to stay out of it, since it's not her place. I'm calling bullshit on it all. DH says I shouldn't participate in the wedding at all, since it'd be hypocritical of me to do so. I am trying to fight off the urge to call this girl and tell her what she's getting into....

WWYD?