A family dynamic where everyone is fucking everyone while keeping it a secret from everyone else.
Little sister has been forcing herself on her poor, beleaguered big sister for years. Big sis doesn't fight back, she knows better than to say no and prompt sis to start whining. She just wordlessly, expressionlessly takes her sister's loads.
Big sis's real obsession is with her (transfem, dyke) daddy. She knows that she doesn't really fuck their mum anymore, and she'd be frankly upset if she did. She's determined to replace mum in daddy's eyes, and so she offers herself to daddy any time she thinks there's no chance they'll be caught.
Mum is suspicious, but has never caught her eldest daughter trying to steal her wife. She's subtly getting her payback though: her youngest daughter is such a needy little thing and almost as hung as her dad, and cums at least twice as much. She's been taking creampies from a girl she gave birth to since she hit puberty and her cock started growing too big for her panties. In addition: her sister's daughter is similarly hung, and the two of them have been staying with the family for some time now.
And yeah, the younger cousin follows the little sister around and does what she does. She double-teams the big sister, and the mother, and additionally sometimes gets her shit pushed in by the dad.
And of course, mum's sister, the horny wine aunt, is angling to take as much dick from her sister-in-law and youngest niece as possible, and really doesn't seem to care if she's caught. Surprisingly doesn't fuck her own daughter much, though.