What can I do in this situation?

Me (M17) live in my house with 2 stepsisters, which I will refer to for privacy as A(F16) and B(F11) . My stepdad who I will refer to as John moved in along with A when I was 13 while B was still living full time with John's ex. As of now, A is diagnosed with many learning disabilities and B is diagnosed with many mental health problems such as PTSD,ADHD etc. primarily stemming from living with her borderline abusive mom (Johns ex). Despite A's learning disabilities, she is still caring, kind and friendly to people although many skills and decision making of hers is off. In terms of B, she is much more abusive, swearing at John, hitting him, and just being all around a bad person.

This doesn't directly effect me (other then disrupting the peace in originally me and my moms home) however there is things that she does that do, I will lead up to the current problems from what I can remember. At the age of 8, B was found putting used toilet paper in the garbage can instead of flushing it (side effect from her mom not letting her flush toilet paper). This can be more excusable then putting used dishes under her bed and where ever else she could instead of bringing them upstairs to be washed when she was 10. She has been caught keeping used pads in her drawers instead throwing them out which caused a noticeable smell in the basement (where our rooms are) this year. And most recently somehow putting human waste in the bathroom garbage can and causing a horrible smell in the bathroom until I found it and disposed of it. These are all more notable events that have happened and there is many others (such as being spoiled rotten and never getting in trouble) that I wont go into today for the aspect of time.

This brings up the reason I am posting this, me, a 17 year old male, doesn't have much control in my house in terms of disciplining my sisters or having any control what they do(same goes for my mother as she is only their stepmom and does not have the relationship with them to properly discipline them without being involved with John, who does not discipline them in the least for anything previously mentioned. John and my mom are both caring and supportive, however the environment with my step sisters is less than adequite to live in and I don't have the first idea on how to make this situation better for myself, other then leaving the house as much as possible and staying in my room when I am home. (apoligies for any spelling/grammar as I am from germany and am not fully fluent in english)

TL;DR - My 2 Stepsisters make my house into a terrible living environment and I don't know what I can do being only their step brother.