Can someone explain the ending

I'm a tad puzzled myself. I'm not sure which harvest they are talking about since they already "harvested" the youngest child from the family.
My imagination led me to think maybe they want to impregnate the daughter, but that's so far into speculation and not substantiated by anything in the movie, that it would be pretty crazy were that to be true.

I can only infer that it's just the typical "the evil is still lurking" ending that all horror-movies goes for.
The only thing the movie tells us, is that the cult want the family to keep living there as they presumably still think they have unsettled business.

It seemed like a pretty clear Rosemary's Baby homage to me. Good movie btw.