Well, I just finished my OB rotation so I'll look in my handy-dandy OB text...According to Adele Pilliteri the risk factors for abruptio placentae are high parity, hypertension, direct trauma, cocaine use and cigarette smoking. There's nothing about previous abruptio, or multiple gestation.
Multiple gestation is a risk factor for PIH though, and PIH is a risk factor for abruptio.
Good luck to your sister! Triplets, Wow, I hope she can relax as much as possible during her pregnancy- because it doesn't sound like she'll be getting much sleep after the babies arrive
Kim