Serena discussion

I like books that keep me wondering after I'm done, and this one has me wondering about two things. First, did Serena's family really die of the flu, or, did she kill them? Second, why did Pemberton cut ties with his mother and sisters? Any thoughts?

I thought perhaps she killed them as well. As for Pemberton the only thing i could think of is maybe they didnt approve of his quick marriage.