Would I be betraying my mom to spent this Holidays with my brother's foster family?

My older brother died of COVID and in devasted. My brother's foster mom and I have started getting close because she has so many memories with my brother. My mom hates her and she got really upset when his foster mom called herself my brother's mom. My brother's foster mom invited me to Holidays with her kid(my brothers foster siblings) I've never met the rest of his foster family besides his foster mom. My mom hates that I spent anytime with her and cried saying I would be betraying her. She said she lost one child to her and doesn't want to lose me too .... I'm really grieving my brother and his foster mom knows him so well