As someone whose family does leftovers (and how!), when leftovers loom the presumed ettiquette is to ask how much you can take (ie. how much does the host want to keep). If it's the best bits that they want to keep, well, that's sad but you're a guest. So taking the best bits? Uh. No.

That said, I agree with everyone saying there's a communication problem between LW and partner and stepdaughter.