I need some advice on how to cheer up my mentally challenged aunt?

Wow, that's terrible! It's good that she finally has someone that will take real care of her. I think that after the drugs wear off, she will feel a lot better and realize for herself that pills are not the answer. But you should try to get her out to do something fun to make her feel better today. Even just a walk will boost her endorphins and make her feel in a better mood. I know it sounds weird, but it works! Maybe you could play cards or board games with her today, since it's raining out. Or bake something yummy! If she doesn't want to, you could always bake something for her? Good luck!