If the guys are getting progressively worse then I think that as a 'nice girl' she probably had low self esteem or was simply shy and a bit lonely. But with some big dangerous looking ghetto black guys she feels excited and alive especially since they sex her up a lot.

I'm black. I'm mostly a geek at heart so I'm mostly involved with the gaming scene and wind up with white women and other blacks used to call me 'Oreo'. however occassionally i still play the 'big black guy' in bed, because it IS terribly hot. The attention, domination and stimulation is just that arousing to some people. especially geeky girls. (i work off the appeal that i'm relatively non threatening but i'm still tall black and can dom when in bed, to get girls) sometimes after being sheltered and being the 'good girl' some girls just want to feel a little slutty and feel a genuinely sexed up, because the attention is intense and addictive and the sex can be spectacular.

if she's finding mostly bad guy's then theres one problem but one thing you'll have to remember is that she's your sister, not your daughter or property. most relatiations as a result of humiliation or anger is just going to make things worse. talk to her as a concerned brother because in all reality what she does with her love life is not your business especially at 20 years old.

This all sounds like a very hot porno as well. i say that because if the sex was safe, and the guy took care of her needs i know a few dozen white women that would LOVE to take her place.