My Dad’s New Wife Banned Me From My Little Brother’s Birthday Party Saying I’m an ‘Outsider’

I (19F) have a 7-year-old little brother who I adore. He’s from my dad’s second marriage, and I always try to show up for him. Last week was his birthday, but when I texted my dad to ask what time to come, he told me his wife said I wasn’t invited. Apparently, she feels like I “steal attention” from her son whenever I’m around, and she wants him to “have family without outsiders.” I’m his sister, not an outsider. My dad didn’t fight her on it, just told me to “respect her wishes.” My little brother cried when I called him that night, asking why I didn’t come. I have no idea how to navigate this without losing him completely.