AITA For refusing to speak positively of my dad and stepmother, when not doing so is causing my stepsister to be bullied and excluded?

I am 20 currently, but my parents divorced when I was 11 and my dad re-married to “Lauren” less than a year later. Lauren already had a baby, Cora.

Lauren was nice before they married, but after she and my dad got married she made it clear that she didn’t want me in the picture and saw me as an obstacle to her “perfect” family.

They would almost always force me to babysit Cora. Oftentimes they would promise I could go see my friends or do something, then I would be forced to cancel last minute because they wanted to go out and needed someone to watch Cora.

When they did spend time with Cora, I was almost always excluded. An example was how I was told to check in on my grandma (which I didn’t mind, as I love her) and help her with chores.

I discovered after returning home that my dad, Lauren, and Cora went to an amusement park without me. Lauren claimed she wanted “family-only” on the trip. My dad did not say anything. This is one of the countless examples when I was excluded and my dad never stood up for me.

At 16, I chose to live with my mom full-time and stop coming to my dad and Lauren’s house. Things are going well. I have a job and am able to stay in the area because my college is online. (Not because of the pandemic-I chose an online program before any of that happened.)

I and a few of my friends were out for dinner when we bumped into Jacob, who was the son of two of my dad’s friends. He and his parents were always nice to me, so we spent most of dinner talking together. Jacob brought up that he was sad that he didn’t get to see me as often after the courts had forced me away from my dad’s home.

It turns out my dad and Lauren told people how my “evul mudder (ridiculous, but also Jacob never met my mom and only knew her from my dad's stories) manipulated the courts” and had their custody completely “stolen” and that’s why I didn’t live with them anymore.

I told Jacob I chose to leave. I named some of the times I was excluded; How Lauren wanted “family-only time” without me and my dad never stood up for me. Jacob was shocked, but we managed to re-direct the conversation and it was still a good night.

It turns out Jacob told his parents, who told their friend group (including almost all the neighbors) about how my dad and Lauren treated me. Now their friends are excluding my dad and Lauren. They found a new phone to get past me blocking them, and are saying I need to “talk about this with them” and “We know you’re a good daughter and will help end these rumors about your family.”

Some relatives are saying I should help them since now Cora is being excluded by her former friends and is extremely upset about it. These relatives are saying that I’m an adult and Cora is a child so she should come first. I agree with putting children first, and I feel bad for Cora since it isn’t her fault. But I still feel she’s not my responsibility. AITA?