AITA for telling my late dad's affair partner I owe her nothing? 

“ how dare you not accept me or the child after I ruined your family?!”

It seems like the AP due to the dad’s passing was now scrambling for support anywhere she could find it. And the wife was lucky they were still married when he passed so she could collect his assets. She deserved it even if they had already filed for divorce. The AP must have been pissed when she realized she wouldn’t get a penny from his passing to help with their new baby. Her best hope would be her own parents/family’s help, but going to the half brother and asking for help, that’s already suggesting they weren’t willing to help either. Definitely NTA, her confronting you at your work after being clear in your stance / choice was wrong too.

For real, AP doesn't want Op to be a brother. she 100% wants to seduce Op and just be the dad. He 19 and the child is very young. Most people wouldn't assume they are siblings and would think he is the dad. Especially if they started sleeping together she would convince him just to "pretend she is your daughter so she can have a happy childhood"

She’s inappropriate and has serious boundary issues. No surprise. If she contacts you again, I’d get a restraining order against her.

You do not owe her anything.. Why didnt she go to his parents to beg for family ties and support?? My guess is she thought shed have a better chance at bossing a 19yr old, who might be easily manipulated; than his long time adult relatives..

Where's the rest of the story? Gotta give you a thumbs down for this. Not even a little bit sorry about it.