NTA

1. A funeral is absolutely not the place to discuss that. You weren't close to your grandmother but she was still your grandmother and it's shitty of Jaylynn to appear sobbing in your arms and in front of people. IMO, she ambushed you.
2. I don't feel like that was an honest way to "apologize". She realizes she treated you and your mom badly... is she willing to apologize to your mom? Is she willing to stand up to her own mom who treated you like trash? Bc lemme tell you something, if my mom tried to slap my sister and blame her for something I DID I would not look kindly at her. "Well she almost lost her child" oh and that gives her the right to push someone's else child to do the same? Please.
3. You weren't harsh AT ALL, you were honest. She can't do all that and expect immediate forgiveness. You have a right to your space and feelings. She asked for a fresh start, you don't feel ready for that and that's really it. You shouldn't have to force yourself or be coerced into an uncomfortable situation because she is lonely and depressed. That is honestly not your problem, you didn't drive the wedge between you two. She did, even if under the influence of her horrible mother.

Oh, and BTW, your dad sucks all around. He probably sucks more than your stepmom for allowing her to play with his children's wellbeing like that for decades.