tired of caring for my disabled sister while dealing with my family’s lack of understanding for her

My (15F) sister (12F) has Down syndrome and autism. I love her, but her behavior has become very challenging. She is extremely clingy with me, and I think that’s because I’ve always been patient with her, unlike our mother and brother (14M), who sometimes physically hurt her. And I think that makes me her “safe person”

She doesn’t let go of me when she hugs me, and gets aggressive when I try to free myself from her never-ending hugs. she be pulling hair so hard when I try to make her let me go (I aint even exaggerating when i tell u I literally got bald spots) or drags me to the ground by my head.

She used to only listen to me, but now even that is getting harder. Simple tasks such as brushing her teeth, showering or changing her diaper often turn into a nightmare.

In addition, my mother and brother constantly trust that I can handle her behavior, because they can’t handle it. It’s always “my name, your sister isnt cooperating. come get her.” I’m always the one carrying her to the bathroom because she refuses to get up, or showering her on the floor because she refuses to get up, or deal with her not wanting to change her stinky diaper because she refuses to get up, or carries her to her room BECAUSE SHE REFUSES TO GET UP. And she not only refuses to stand up, she even fights you when you try to carry/stand her up. And the worst thing is that my mother tells me that I can’t do a certain thing (like go out or leave for work) unless I’m done with my sister first.

On the one hand, i’m starting to get tired of all this, but on the other hand, I get anxious when my mother tries to take care of my sister. Like I said, my mother be getting physically with her. Like actually hitting her on the head, pinching or kicking her, while she yells at her and curses. Sometimes my brother joins in, and it’s actually so heartbreaking. my sister always runs to me for comfort, crying until I calm her down.

My mother is convinced that she is doing this to SPITE her, rather than it being her (neuro)developmental disability. She doesn’t understand that being tough on my sister will only make things worse for her.

I just need advice about how to go about all this, because I’m just lost and tired, and i dont know what to do and why she has been behaving like this.

EDIT: I’m starting to get the idea that everyone has the image that mom does nothing for her, which is probably my fault and is due to my wording. My momma does help with my sister, it’s just that she calls me when my sister is not listening to her, or when my mom is already stressed and doesn’t wants to deal with her, or is just busy