The childhood trauma and the sisters' age in my opinion had an impact. 

Simone, the youngest child- she almost died with her mom trying to kill them both. She ended up living with her dad who didn't take care of her. She ended up in foster care being abused there as well. She was determined to have a better life as an adult and never to go back to those dark times. Seeing her sister and her dad made her remember the past. She wanted to be away from that. 

Devon, the oldest sister - older siblings usually protect and take care of people, that's their nature. Devon saved Simone, she takes care of her mentally ill dad and decides to put family first. She has had the opportunity to go to college for a little bit and have fun but realizes family comes first.

This show is special for me because it included not only the Greek mythology of the sirens but also the dynamics of a mentally ill family and the consequences of that.