Responses
it doesn't look like she love her bio sons honestly, she's just that blinded by range and revenge
You're probably right when it comes to love, but they do represent her victory over her husband. Kill them and she has lost, the king has won, in her mind. If revenge is all she cares about, take away her revenge and make her watch the ones she hates be victorious.
Couldn't you just kill her two real sons and her brother in front of her as punishment? Then the only blood alive would be the blood of the mistress and she'd have to be alone the rest of her life. Much worse fate tbh.