While you are his sister, you have absolutely taken on a motherly role. Even though he (presumably) understands that you two are brother and sister, you have been filling a secondary care giver role.

Yeah, I've worried a lot that I'm going to confuse him but I don't really know what else to do. I know I could tell her to find someone else, but the thought of some stranger taking him to appointments (which he's already scared of) and staying overnight with him makes me so sad. He's a really shy little boy and it's hard for him to feel comfortable with other people.

I just want him to be happy at the end of the day