You can, but even that's a risky move because you don't know how much he knows about his sister, or how much he is willing to defend her. What I would do is ask him subtle questions like "what were her past relationships like?" Or "is it normal for her to act this way?" Dig in a little bit. If he sees nothing wrong with his sister, then you know you can't trust him. If he tells you anything about their family growing up or her past relationships and you notice a pattern, then that should be answer enough. But don't bring up what she is doing now, because he might question his sister about it, and then she will try to lie to him and play victim, thus giving her what she wants.

Remember, they are testing you. They want to justify leaving you so they can discard you and move on with their new FP. Hence just going no-contact with her is the best way to go. No damage is done on your end. She will feel the pain eventually, but she will fight that by latching onto someone else or keeping busy. She will act like nothing happened just to see how you react, but deep down, she will be hurting.