Would you stand by and watch someone push someone else off a cliff and say "Oh, no, I can't interfere, that's family business"?

Let me try this again in small words. You are (supposedly) the adult. Rage and scream and pout about how she's hurting your feelings all you want, but do it at someone who can help you work through those feelings and refocus on helping her. Don't do it at her. She is the one who needs help - you don't get the privilege of using her as a punching bag for your own insecurities and unhappiness.

People in need don't always want help, Piotr. A lot of the time they reject it, sometimes violently. You owe her more than to let her defensive mechanisms drive you away.

Grow up.

Nathan