You can't trust strangers in these times (especially in the US it seems). So I asnwered their questions very vagely. Later I thought they were actually bandits/prisoners, who killed the original owners of the farm (until I saw the picture of the lady and her husband in the house).
Hearing about their deals with the bandits also made me quite alert.
In the wood, when the other guy shoot the crazy woman, I knew that they're up to something. And when the lady didn't let me see Mark, I wanted to grab a gun and shoot them all, but the game didn't allow it.
So ... I thought that they might be dangerous at the beginning and then my suspicions just grew stronger. But I didn't expect what they were doing, until I've seen the slaughterhouse in the barn. That was really quite surprising.
But in the end I didn't kill the brothers. I thought that the walkers or bandits would get them as wounded as they were. That seemed to be a good solution and you don't make your hands dirty in front of the kids and can actually play the good guy.