Is my boyfriend's younger brother spoiled or am I just being unfair?

I moved in with my boyfriend recently. His younger 28 year old brother stays with him. It's a two bedroom apartment. My boyfriend foots the bills, rent, food, utilities etc and I will too in a few months when I get back on my feet as is the agrrement. His younger brother does not work a formal job, he is a music producer (who produces mostly for free and for the love of it) and who works from home as he has been scared of being rejected job-wise his whole life as he has a medical scalp condition which requires him to wear a cap all the time.

Anyway long story short, my boyfriend is getting upset when I say I will no longer cook if his brother is going to leave the kitchen filthy like a pig. When his brother went out the country on a trip for 3 weeks I cleaned the house from top to bottom because it was filthy. The house stayed clean while he was gone and when he returned it returned back to being filthy, especially the kitchen and I mean pig stye filthy. My boyfriend ends up cleaning his younger brothers mess even though he works hard just to pay all the bills and I end up feeling bad especially because my boyfriend gets upset with me for not just doing it. I tried at first but his brother continues not to be considerate, if anythings its become worse after I clean up. I mean I cook I clean all the time already (morning noon and night), I leave everything spotless, but I cannot be doing it 24/7 just because his brother makes filth whenever he feels like it, no matter how much you wash up after him. So yes, I'm will not clean his brothers messes. Yes he has a medical condition, but I feel if he can hitch hike to other countries and have fun in the studio from a.m to a.m then he can be hygienic. I feel he is spoiled and I will end up taken advantage of as well as an being enabler like his brother. Besides that he is snide and secretly nasty to me when people aren't there and yet I try to be nice. Am I wrong for my sentiments and how I am going about it? What do you think should happen in this situation? I ask because I can tell my boyfriend is frustrated for having to wash up after his brother because I won't budge and he does so because he says his brother just doesn't pull his weight no matter how much you tell him.
Is my boyfriend's younger brother spoiled or am I just being unfair?
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