AITA for giving my niece a better life than her step-siblings?

I’m a 40-year-old woman who’s been incredibly fortunate in my career. I joined a tech startup early on, and it grew into a unicorn. The company was sold a few years ago, and while I’m not a billionaire, it’s provided me with a very comfortable life. I have a small, close-knit family, and I’m happy to share my good fortune with them.

I have an older brother, Isaac (50), who has a daughter, Grace (14), with his ex-partner. Grace primarily lives with her mom, Rebekah, but spends weekends and half of the summer with her dad. Grace and I are especially close since I only have sons, and they adore their cousin, just as much as she loves them. When Grace was younger, she would often babysit for us, help with the baby, make bottles, and be an all-around helpful "mother’s helper."

Even before my startup sold, my husband and I made enough money that we paid for Grace’s private school tuition (about $60k per year) and took her on trips to places like Disneyland, skiing, and even vacations to places like Mexico and Hawaii. But once our lifestyle upgraded after the sale, so did Grace’s. We set up a trust fund for her that covers her college, master’s, and PhD tuition, and even a down payment on a home in the Bay Area. We also bought vacation homes and started taking frequent international trips in business class. Her parents have been incredibly supportive and very grateful.

The problem arose about five years ago when Rebekah met George, who had two children, Caroline (16) and Christian (13), from a previous marriage. George has 50/50 custody of his kids, and both he and Rebekah work as servers, making it difficult to make ends meet. They recently got married and moved in together. Unfortunately, Caroline and Christian have started feeling jealous and upset that Grace has a much more privileged life. They go to public school, get a trip to Disney once a year (if they're lucky), and maybe a trip to visit their grandma in Florida during the summer.

This led to some tension. George got upset, which caused arguments between him and Rebekah. Caroline and Christian have been upset as well. Rebekah, hesitant but feeling obligated, brought this up to me. She told me that George asked her to speak to me about at least helping pay for private school for his kids to "even things out" between the siblings. She also mentioned that George wants me to help level the playing field with gifts and vacations.

Rebekah told me that she knows it’s not my responsibility, but she promised George she would at least ask. She also said George had called me a "cold jerk" for treating the kids so differently, but I didn’t feel that way. My husband and I are on the same page—we feel like I’m under no obligation to treat Grace’s step-siblings the same way when I barely even know them.

So, AITA for refusing to help with their private school or for not treating my niece’s step-siblings the same as her?