I'm so sorry for what you're going through because that sounds really tough, dealing with teenagers and only being married/with your husband for 4 years.
However, you said something that kind of jumped out at me, you said "It's like she comes before me and that's not ok". She does come before you in her father's eyes and honestly that's the way it ought to be. Children always come first and its even more obvious when it comes to step parents.
But your husband is not handling it correctly, he is doing what a lot of guys would do by trying to ignore or not escalate the issues and sweep it under the rug. My dad turned a blind eye to all the horrible things my mom was doing until it was world war 3 in our household. I'm not saying all guys are like this, but a lot of them are - they don't want to rock the boat and it has to do with fear.
My mom always said "You girls always come first." Then when she started cheating and met her boyfriend she said "The man is always supposed to come before the children because you can always make more children but you will always have your husband/boyfriend." Yeah talk about being dumbstruck in love.
Anyway...if its really that bad then yes you can leave but I think there needs to be a talk between you and your husband about how you're feeling, and then there needs to be a talk with the entire family (you, him, and the girls). If things don't improve then obviously you need to do whats best for your life, even if that means leaving.
Yes you will have to begin working again - can you work and support your son with your current health?