Salaam,
It is our Muslim duty to point out that which hurts others or is unbecoming. However, it depends on your relationship with your sister in law. Often, many in-laws become defensive and will tell you to not interfere. It would be more beneficial to talk to your brother. Why does he tolerate verbal abuse? Or are you seeing it as abusive and he is not upset by her behavior? Talk to him first. I pray this helps.