Where i come from messing with schizophrenic people like that, saying things are a hallucination that are not, or acting like a hallucination is real is considered abuse/neglect. Its actually very dangerous and could significantly worsen her condition… she needs to get out but its quite possible she psychologically trapped with your brother. As sad as it may be saving her isn’t your responsibility to carry though, but if there friendly connections that could help, she may need someone more mature looking over her.
I asked my partner for an extra opinion on your “manic episode” it doesn’t seem all that out of the ordinary. Manic episodes tend to last hours. This just Appears to be a temporary state of mind.
What i think could more accurately explain it is that you where overloaded and exhausted by sensory experiences which can be physical experience like light and sound but also mental ones like remembering a long to do list, receiving new information. Its very common for autism.
Different from what society seems to assume many autists are very emotional beings but we often struggle to express those feelings or to connect them with neurotypical emotions. We tend to learn automatically to exercise the self control to contain or genuine pure feelings but when we are drained those emotions leak, which can actually be a really good positive moment of reflection and selfgrowth… granted if your with good people willing to listen in kindness.
I hope your in a better spot with better people where you are now. Being family i assume it may be hard to really get away from those people but i wish you strength in keeping their nose out of your mind.