Some heavily mentally disabled aren't aware of their existance

My sister and i had an argument about this. I think that there are some disabled people that lack the necessary mental capacity to understand that they're a being that exists in this world. I basically think that they don't possess an awareness of self just like how some animals don't. She thinks the opposite and believes that every human, no matter how disabled they are, actually knows that they exist but simply isn't capable of showing it. She thinks that my viewpoint is absolutley generic and that I belong to the dumbest People on earth for it and suggested that I should never tell anyone else that i think this way. Perhaps she's right, perhaps she's not. I thought that this sub is the adequate place to post this because apparently no one would agree with me.