https://www.reddit.com/r/movies/comm...of_becky_2020/
I think u/kirinmay was on the right track.
At the beginning of the movie, Becky is doodling the SS bolts over her name on a test which is a Nazi symbol.
At the gas station Becky looks around at the white people in the family reunion shirts and stops to stare at the one non-white looking woman then leaves the store to wait outside while her dad shops.
When they get to cabin, Becky starts off "She wouldn't want us to sell it. Just because she isn't here anymore..." and her dad cuts her off that he'd changed his mind last minute not to sell it. This would seem to imply the cabin was more her mom's than her dad's.
When her dad says he's getting married to Kayla, Becky looks at her with disgust and blurts out "She would have hated you!"
The Nazis have been in prison 9+ years and the literal key to their Nazi plan that they've been working on for years is hidden in cabin where they know it is in the basement.
When the Nazis get to the cabin they don't know Jeff or Becky at all, nor do Jeff or Becky know them.
Jeff was unaware of the key or the Nazi plan. Becky was aware of at least the key.
So my theory is that the dead mother was a Nazi and working with them on their plan. The plan had been in the works for much longer than she had been dead. She was keeping the key for them in the basement of her cabin. The Nazis found out she had died and the cabin was going up for sale so they orchestrated their prison escape to collect the key to move forward with the plan before someone could buy the cabin and trash the key.
Jeff possibly only tolerated his wife's racism (not knowing her Nazi affiliations) or was himself racist until he fell in love with a black woman after her death. Becky knew more about that side of her mom, hence her doodling Nazi symbols, appearing bothered by mixed race families, and knowing of the key to the Nazi plan.
However, since the Nazis kill her dog and threaten her family before she even knows who they are she takes action against them and kills them all out of revenge. She then keeps the key for herself rather than telling the authorities about it.
Is Becky racist? I think she has picked up some racism from her mother. Is Becky actually a Nazi too and keeping the key to continue her mothers plan on her own or is she just holding onto it as some morbid keepsake? It's hard to say.