Simple formula for success
I have come up with a solution to my issue. Seeing as I'm in a situation I cannot possibly win, I have to deal with any principles of self-respect, fairness and sticking up for my girlfriend, simply by forgetting I have any.
Her kids can now treat her how they feel like, unhindered by any input from me.
She is treated with nil respect, snapped at, walked all over. I am sick of trying to intervene as whatever approach I take seems to be wrong.
She can feel guilty at hurting their feelings by attempting to instill decency and respect.
Sorry for the rant, I have nowhere else to turn.