HELP , ADVICE and GUIDANCE !?

well , i have a brother who's girlfriend is quite insane i think , wel i personally think there's something mentally wrong with her , anyway when our family first met her we thought she was alright till her and my brother had to move in with all the family , it didnt take long till we all started realizing how she really was , it all started when one night she grabbed my two nephews who were playing in the lounge where her , my brother , my partner , my sister and i were all sleeping , she dragged them into my sisters room , told my sister to look after her kids then she slammed the door and started on me and my brother , my sister ended up coming out of her room and started yelling and arguing with the sister inlaw , by then all my family were all in the lounge trying to understand what happened , my brother ended up taking his girlfriend outside but when they were walking out his girlfriend turned to my sister and yelled '' I HOPE YOUR BABY DIES ! ! '' .. Thats how everything started .. And there is more that my sister inlaw has done to split my family apart .. PLEASE HELP !