Did mother know that Sis stabbed Dad?

After sis has her brain extracted from, she stabs Dad. The reaction that mother has to sis stabbing dad seemed a bit strange. I know this is a surrealist film, but to go from dead Dad to let's burry him seemed a bit rushed.

Did mom just assume she accidentally stabbed him?

Absolutely loved the film, but that one
scene kinda confused me

Thanks!