My first thought was that the brothers were somehow in league with the bandits -- that the confrontation in the woods on the way to the farm was a setup for our group's benefit, and that all the talk about "safety" was intended to lure our group to the farm for exploitation of some sort by a larger group. I also noted (and confirmed on my second playthrough) that while Andy claimed he turned the fence on AFTER Mark yelled, in fact Mark was shot after the fence was turned on.
But I hadn't yet gotten to Volume 11 of the comics (just did yesterday), so cannibalism wasn't the first thing that came to mind. I was thinking something more like capture and enslavement to keep the farm operating -- we could see enough deterioration to think that the family was operating more out of need than kindness. I didn't start to think cannibalism until I started to notice that the "dinner" repeatedly mentioned was remarkably non-specific, and then of course the back room in the barn made the nature of the threat clear.