You ever see people complaining about fathers talking about 'babysitting' like that's an imposition and not their flesh and blood?

Being in the room with your infant sister for literally 10 minutes is not 'unpaid babysitting labor' like it's an imposition, that's your flesh and blood.

People cheat, people get divorced, it's often messy, being a resentful teenager about it doesn't help anyone, that's still your family. The father is still supporting his daughter and I doubt the pregnant stepmother who still 'does everything for you' is getting a fair telling from a resentful 16 year old in this story.

It's totally understandable to feel resentful about this and it's honestly not unusual or a big deal in the end, but yeah YTA for not literally just being in the room with an innocent baby for a woman who supports you regardless of your feelings toward her. More accurately, you're the teenager. But they often overlap.

When you're older, you'll benefit a lot more from a good relationship with your stepfamily than from the gratification of your moral outrage about your father cheating. Marriages fall apart sometimes, it sucks, it's a fact of life. A father who is faithful to his (first) wife and negligent or abusive to his children would be a lot worse than it sounds like you have it.