I don't know about Pitbulls, but my Sis-in-Law's Argentinian Dogos(part Pitbull) are very aggressive towards other Dogs. Even with good Training they need to not just be watched, but to be leashed when outside a fenced in area. If you need to ask all these questions, I really think you should consider another Breed. These aggressive Breeds need a lot of attention and they will kill other Dogs if you are too casual with them.
Unless your sister's were specifically bred with pitbull, they aren't part pitbull.
The majority of pits, like most dogs, are well behaved. All dogs are capable of injuring people, and most breeds have killed someone at some point in time. All dogs should be watched when socializing with other dogs, and should NEVER BE LEFT ALONE WITH CHILDREN.
Pits have a bad name, and any medium to large breed+ dog with poor training will be dangerous to a degree.
I've got a 98lb American Bulldog pup (14mo) and he's great with other animals and people, but I wouldn't leave him alone with a stranger or child or dog that he hasn't spent significant time around. He's leashed every time we go outside. It's just best practice.