(revised) When her mother is brutally murdered, a family woman must find out who her mother was, in order to uncover what happened; but when a cult from her mother’s secret past starts to infiltrate her life , she must save her family and herself from the cults’ grip or risk becoming victims themselves.

Seems better... but I'm wondering if this can be simplified to just jump to the cult.

After learning of a secret cult's responsibility for her mother's brutal murder, a woman must now save herself and her family from being their next victims.

I think that gets to the interesting part more directly and still remains true to the story, no?? I don't think it's important to make explicit how the cult operates.? I do like the mother "investigating the mother's secret past", but I don't think that's part of the hook for this movie.