How do you get a dear aunt with mental health issues to open her heart to you? We all loved her, but she is distant and abusive towards us.?

you don't, you try to love her the best you can.....

Mental illness can take a variety of forms, so this is almost impossible to answer. All you can do is to give her your love; people do respond to love in one way or another, even if we "normal" people cannot recognise it. 

It sounds like your poor aunt is unaware of the fact that she drives people away with her behavior.

All you can do is wait for her to contact you and be there for her but not stressing yourself out too much.

You can lead a horse to water but you can't make it drink.  In this context that would mean that you can't force someone to be more communicative but you can keep trying to encourage them to.  Someone has to want to change if they are actually going to.  All you can to is to try to help them want to, and if they don't you have to respect that.