He's the one that won't bootleg for the kids, but he'll let them know how to get it without him. He teaches them how to pick up girls, hang out in strip clubs without looking like a douche and when you need to talk about stuff that you can't go to Daddy with - you go to the Bad Uncle.
Bad Uncle is also the guy who you need to worry about when it comes to his girls. The girls Daddy might talk tough and all, but the Bad Uncle will fuck you up if you hurt his niece.
Bad Uncle: Wait there don't say anything and I'll be there in 10 minutes. Don't call your Dad.
Niece: Oh Uncle... I met this great guy last night he's so nice...
Bad Uncle: Uh huh........ Yeah I'm going to need to meet him. Did he try anything?
Niece: No of course not.... Like I said - he's niiiccccee.
Bad Uncle: I've told you since you were 5 minutes old - Never trust the nice ones cause you don't know what you're getting. I'd rather you meet a prick because then I know what I'm dealing with.
If you want to see him again - I'm going to meet him first.
Niece: But Uncle...
Bad Uncle: But nothing... and if he hurts you they'll just never find the body