Currently my brother who is younger than I am. Backup plan would be my Trust.
As a matter of fact, I am currently assembling my "Ok Now I'm Dead" binder/online, and going through all my papers to organize materials for my executor. It is one of my goals in the first year of retirement. Does everyone have something similar?
My Mom did, my Dad does, and I do.
My Mom's had extra stuff in there, but it's better to have too much than too little. She also did a thing where she made sure we all knew about "the green notebook" and where all the papers were and stuff. Also, the last year or so of her life, she had a box where she put anything important. When I came over to visit, she trained me to go through the box and review all the papers so I knew what was going on.
As far as OP's question:
My Dad has three kids, and the executor/trustee role is filled by age. So my oldest sister is primary, other sister is secondary, and I'm tertiary. I'm the designated medical/POST person and I have a durable POA as well. The former because my Dad thinks I'm more likely to pull the plug per his wishes than my sisters would be, and the latter is probably because I'm local and my sisters live out of state.
In my own case, my middle sister is my executrix. I picked her over my eldest sister because she is younger, lives closer to me, and is better suited to the role.
I wanted to switch it to one of my children. I discussed it with them and for various reasons this idea is problematic, so for now I'll leave it with my sister. Maybe I'll revisit in ten years or something.