Harriet DIDN'T KNOW what happened to her husbands!!!
I've seen various posts in different topics on here that refer to Harriet knowing about Rose killing her husbands (e.g. the reason she originally didn't want to get back together with Charlie was to protect him from Rose... the nightmare where she screams "Ralph!" she was dreaming about Rose killing Ralph etc).
She didn't know!!
The reason she acts so weird at various points in the movie (especially once they get to the hotel) is because she thinks men keep leaving her.
When Charlie breaks up with her at the park she says "At least YOU left early on."
When they are at the hotel she watches him like a hawk because she thinks he's going to run out on her.
Charlie even says in his final poem "You acted cuckoo 'cos you thought I would leave you."
Oh yeah... and the fact that Harriet is shocked when Rose starts attacking Charlie.
Charlie: Rose just tried to kill me
Harriet: WHAT!?
IDK how people can think she knew all along.