You need to set a very firm boundary. Tell your aunt that you cannot do it. Tell her "no." My therapist has told me many times that "no" is a complete sentence.

Will your aunt get upset? Probably. But that is her issue. Not yours. If she demands an explanation, tell her it is not feasible with your housing space and that you and your partner don't want a roommate. Even if it is a sibling.

My brother has a lot of problems. I have my own mental health issues. Us living together is not an option. It would be a bad situation for me.

It's OK to say no. *hug*