AITA for telling my brother if he doesn't allow his stepdaughters to come over for Christmas dinner, then I am not showing up?

I (28 F) have a brother, Michael (30 M) who has three daughters, Chloe (10), Heather (7), and Laura (5). He also has two stepdaughters, Nicole (15) and Rachel (13). His wife Janice (29 F) is their mother.<br><br>We always go to my parents house for Christmas dinner. My mother always invites Michael's family and Janice refuses, saying that Michael should spend Christmas with his daughters. My parents invited them again this year and Michael said the girls could come, but he and his family won't. My mom asked if Janice would be okay with Nicole and Rachel coming and Michael said Janice isn't their mother. My mom said that it isn't fair to leave them out of Christmas and Michael is entitled to spend it with his daughters. My mom also said they (the stepdaughters) have no family if they aren't their family. <br><br>Michael and I were discussing Christmas and I told him if he doesn't let the stepdaughters come, then I am not showing up. He asked my why and I said that it isn't fair to leave them out of a family holiday and treat them like they aren't his family. He said that they aren't his family, only his daughters are. I said Nicole and Rachel have no father and need one and it's his job to be their father. He said all he owes them is a roof over their heads and food and that's it. I said that's unacceptable and he said Janice agreed with him.