My brother is the admin of all the computers at my mom's house, and he actually created a password for her and forgot to tell her what it was - she finally got it.
He also set my computer up for me several years ago, and I later found out that all my emails were going to him AND me. I was going through a divorce at the time and I was furious.
He also monitored my kids' use of the computers at Grandma's house, and would shut them down or make comments if he didn't 'approve' of the site they were on (FB/MS). He'd shut down the network at 10pm because it was his bedtime - which affected me, an adult.
He used to spy on me (with recording devices, etc.) when we were younger, but the computer stuff really caused a problem. He was not entitled to have access to my personal data, but he decided he was.
Guess that's why I had such a strong reaction. My mother has her pw posted on her desktop, but he gets into it remotely. I do not like bringing my computer there - I don't trust my brother. It's very sad. He never gave me the admin password to my OWN computer... I just had a user account. I did NOT ask him to do this, but that's when I trusted him and assumed he would be helpful because he really knew his way around Macs.
I still don't know why he did this things, but he was really into covert surveillance type stuff, and I think it was just - he enjoyed being able to do it... pretty unsettling.