The bathroom my brother left me to come home to from college, that my parents want me to clean

My 18 y/o brother lives at home while I am at college. He is gross, and doesn’t clean up after himself. I am a very clean person who likes a clean space. Every time I come home, our shared bathroom looks like this. Normally I clean it because I can’t stand it, but this time I decided I’m not going to. I’ve been showering at my girlfriends and pooping downstairs. I keep all of my toiletries in my bedroom. I politely asked my brother to clean the bathroom several times now and he won’t. The other day I asked my step-dad politely if he could ask my brother to clean the bathroom since it’s gross. His response was that my brother isn’t going to do it, and even if he asks him to he won’t listen, and since my brother is now an adult my step-dad doesn’t care that he won’t clean it. He said “why would he clean it, he can stand and live in the filth.” And “it doesn’t inconvenience him, it only inconveniences/affects you.” My step-dad told me if I want it clean, I will have to do it myself. He got mad at me "bitching" about it because I don't do anything around the house but my brother mows the lawn. I told him he can give me a chore to do and I will do it. Dishes, sweep, etc. but I'm not cleaning that bathroom. I even offered to clean the bathroom every week for the rest of the summer if my brother cleans it the first time very good. He still said if I want it clean I will have to be the one to clean it. My brother is a literal adult, and I feel he should be the one to clean his built up filth. I feel my living space has been disrespected and I honestly feel disrespected as well. I find it unfair. Plus, if I were to clean it, he would just filthy it up again in a week. He has no respect for the environment around him. I also mentioned how I knew this would be an issue before the summer started, and I discussed it with my therapist. I told him I made a boundary with my therapist and he said, and I'm not kidding "go shove it up your therapists ass and go live with your therapist then." We got in a screaming match about it going in circles. He said it was BS that I won't clean it since I don't do anything around the house, but again I have no problem doing chores, I do however have a problem with scrubbing my adult brothers shit off the toilet. Am I in the wrong here? Both my parents think I should be the one to clean it if I want it clean. I just think they are insane and have very flawed mindsets, but maybe I am in the wrong?