<human>: Give me a shortened version of the text

My stepsisters are at their wit's end with their biological mother. She is this exceptionally controlling, narcissistic woman who makes their lives a living hell. They've been extremely vocal to their mother about wanting to live with my stepfather full-time because they have no desire to live with her anymore due to her behavior. 

Tonight, there was an incident at their mom's house where my younger stepsister got in a huge argument with her mom, so she called my stepdad and asked for him to come pick her and my other stepsister up. Their mom ripped the phone out of my stepsister's hand and told my stepdad if he comes to get them she will call the police and say he kidnapped them. My stepsister then texted my stepdad and said she took all of her stuff, even the picture of her and my stepdad that she keeps by her bed. She told her mom she hates her and never loved her and doesn't want her in her life anymore.

These two girls have been in my life for over ten years now, and I consider them my real sisters at this point. The youngest grew up with me since she was a newborn, and I was in the older one's life since she was two. Whenever they tell me about how awful things are at their mom's house, I wish there was something I could do for them. This women is honestly psychotic and will do anything to keep the girls in her custody, even though she is mentally unfit to be a parent, in my opinion. 

What can my stepdad or my stepsisters do to get out of this situation? Tonight's events have led my stepdad to consider taking legal action because he is tired of her behavior and fears for his daughters' health/safety. What can he do to gain full custody? Are my sisters legally able to decide where they want to live?
<bot>: My stepsisters got into a huge argument with their biological mom tonight and want to live with my stepdad and biological mom permanently. What steps do my stepdad and stepsisters need to take to change their living arrangements?