My brother (14) and stepsister (12) both require full time care because of their autism. Both are non verbal. My brother has the ability to do some stuff for himself but he can't ever care for himself and one day he'll need to be placed in a proper facility that can provide care for him. I (18M) always knew it would fall on me to take over guardianship some day and to make sure he's in a good place and cared for well and to visit. My younger sister (13) has always promised to help me but neither mom or I want it to fall on her. My dad died when my brother and sister were really young so I have always tried to help where I can.

When I was 13 my mom met my stepdad. He had two daughters and his oldest had autism too. She's a lot like my brother but has not got the skills my brother has and she requires a little more care than my brother.

There was a difficulty from the start where my stepdad hated seeing me help with my brother and care for him so well but I never offered to do the same for my stepsister. He would ask and I'd usually say upfront I couldn't go from helping with my brother to my stepsister because I needed to focus on school and I needed down time. He asked me to choose my stepsister instead of my brother at times and I told him I couldn't do that. He told me his younger daughter (9) is too young to help out and I told him he'd need to figure out an alternative.

At some point he and my mom talked about what would happen to my brother when mom's too sick or dies and they somehow came up with the idea that because I'd take guardianship of my brother that I'd do the same for my stepsister. I only found out about this when my stepdad mentioned the fact I need to get to know my stepsister better so I can know how to advocate for her better. Because it wouldn't be fair for my brother to get better care because I know him and what he needs. That's when I was like wth are you talking about and then he brought my mom in and they said they expected me to do it for both. Mom asked me could I honestly just make sure my brother's okay. I told her yeah because he's my brother and it's a big responsibility that will require me to keep watch over everything and to make sure he's being treated well and staying healthy and will mean being with him when he gets sick and stuff. My stepdad said I'm perfectly capable of doing it for both when I won't be taking care of them day to day and that it was disgusting that I'd let his daughter rot. I told him I won't let his daughter rot. That if he refuses to find someone else that'll be on him.

They're really pissy about it so now I'm wondering AITA?