Should I speak to my sister?

My dads an addict. He hasn’t gotten treatment like he said he would so we’re going to do an intervention.

He has a strained relationship with my brother and quite frankly I think they both hate each other. My brother hasn’t wanted to be involved because he’s embarrassed over the situation but after our first meeting said that he’d like to maybe write a letter(we’re all writing letters).

My mom spoke to my dads sister after the meeting and mentioned that my brother was thinking of writing a letter. My aunt immediately shut it down saying it wasn’t a good idea, my brother hasn’t wanted to be involved, that it’ll just make everything about my brother and that their issues are a whole other ballgame. My mom basically told me that she’s not going to let him write a letter after what my aunt said.

I feel bad and feel like I should speak to my sister about letting him participate if he wants. The only problem is, my mom feels like my aunt and sister are on the same side so to speak(after the meeting they got together for a drink and didn’t invite us. According to my aunt, my sister needed it) and that they both feel like my brother caused my dads issues.