Need a bit more information. For example is your sister aware you know about this? Is she likely to honestly tell you what's going on if you ask her, or would you be confronted with protestations of "YOU DON'T KNOW HIM LIKE I KNOW HIM"? Because the latter obviously makes the situation about ten times trickier, and in such a case the first thing I'd do is ask any mutual acquaintances if they've noticed any abuse or strange behaviour from the boyfriend.