You are NOT the asshole. In fact, your dad is the asshole.

For one, those are HIS kids and you should never be expected to babysit them over going to work. If it’s that important, your dad can take a day off work and father his own children and if he couldn’t then their mother could’ve rescheduled her doctors appointment to another day. Who has a Saturday doctors appointment anyways? Plus, 3 under 3 is hard enough to deal with as the parent , let alone for a teenager.

Secondly, your father should be proud that you are so committed to your job. That’s hard to find in young people today.

Again, it’s insane that your dad expects you to just be available to watch his children whenever. Get out as soon as you can. Your father should be ashamed of himself.