nah you're not wrong, I'm also confused about that arrangement he has with his sister (I mean the sisters a psycho anyway)
The sister gets the money he gets money by his shop he is doing it mostly on a trauma thing and he feels he need to cuz his sister took care of him only to turn in to the parents
His sister is pimping him out, she's probably doing the same to herself, for money. He said that his sister helped him opened the tattoo shop. Now with their trouble background, where would they get that kind of money easily ? Sex work. In her unstable mind, she probably thinking she's doing him a great favor by providing him with a source of income to live comfortably. And Mitsuki still continues to listen to her because he thinks he own her for all she's done for him + being his only family + only one who cares about him at the time.
I really don’t understand what this arrangement with his sister is. Like is this a sex work thing? Is he getting paid? Am I the only one confused about the arrangement? I feel like it wasn’t really explained.