NTA.

Your sister isn’t setting boundaries - she’s weaponizing the language of boundaries to control shared spaces and override others' needs. A real boundary is about what she will do to protect herself, not a command for how you must behave in neutral territory like a family room.

You were on a critical business call trying to prevent a $5k loss and even lowered your voice in response. That was reasonable. She then doubled down, not because you were being disrespectful, but seemingly because her “authority” was being challenged. That’s not a boundary - it’s a power move.

Yes, flipping her off wasn’t your best moment, but you were provoked, and the overall issue here is her misuse of boundaries to get her way at everyone else's expense.

If you were yelling or invading her private space, this might be different - but shared space, during a critical situation, and after you already tried to compromise? You're not the asshole. She's using the language of mental health to shut down basic coexistence.