What would you do?

My parents are older and infirm. I spend a lot of time caring for them, especially when my mother is at what I was led to believe was a support group for spouse caregivers. This weekend, they went on a trip to visit my aunt, and I agreed to look in on the house, get the mail, water the plants, feed the cat. When I walked in today, the first day they're gone, with the mail, there were papers all over the table. I was straightening them up, and I'll admit that I looked. They were worksheets, from what is in fact her behavioral therapy group, enumerating in painful detail how much she loves my sibling more than me and how I am a problem to be managed. I am totally devastated. I love my parents, moved back to my hometown, am underemployed, and over their house every night. I can't understand what I did to make her hate me, and I am heartbroken.

Edit: I did Google the therapy place, and their specialty is bipolar disorder. She's never discussed a diagnosis of any kind with me.