I agree with you, the thing you mentioned is so common in Asian household I think a lot of ppl who grew up in this type of household would relate & understand her course of action & frustrations.
That being said, of course she shouldn't outted her brother like that but I know that it was the only move she had so that her mom would look at the brother more as a human being with faults, equal to her, instead of this illusion of perfection.
Cool, so because she's not mom's golden child she can strip her brother's choice to come out safely? She didn't know or care if her mother was going to completely reject and disown her brother. Just because the mom watched some bl drama doesn't mean crap since people can simply see from an outside view and never tolerate it themselves. She's selfish as hell and is complete trash. I'm saying from a point-of-view of queer person herself that also have queer relatives that also find it hard to fully come out to their entire family.

I'm not saying the sister is justified. But as an Asian with a mother who raised a golden child son and escape goat daughter. And brother who sees nothing wrong with this relationship dynamic. I'm just saying I understand. Don't expect anyone to be a Saint when you treat or watch them get treated as garbage.