A lot of us bipolars run hot&cold when it comes to meanness&being loving. It can be a part of a manic episode-mania doesn't always manifest as delusions of grandeur, shopping sprees, or hyperactivity. We may manifest mania as irritability, meanness, cruelty,etc.
Remember she is sick, but you DON'T have to put up with her crap! She sounds like my mother-nice one minute, nasty the next with a lot of backstabbing&destruction of my personal property. I just don't put up with her anymore. Your Aunt is family, but there are reasonable limits&you're not being mean if you put your foot down&say I'm not playing your game anymore.
Like myself, you seem to need to not take an obviously mentally ill person's cruelty/lies personally. I have a real hard time not taking things personally&I'm even worse if there's criticism. It's to the point where I am hurt so badly inside that I can't tell if ANY of what they're saying has truth to it.
hang in there-I'd say stay away from her until she starts acting nice-I know it's confusing&unpredictable, but sometimes nice is better than NEVER nice.