I think there's a difference between a paid babysitter and a family member first of all.
Second, if the dad spent his whole life doing work on this type of thing, he really shouldn't have been surprised that the same was expected of his tween. And he likely thought that the tween could deal with it. When I was younger, I used to stay with my grandparents for a couple weeks every summer. I did a lot of the housework, helping to cook, did the vacuuming, dusting, etc. And you know what? I did it at home too, so why shouldn't I do it at grandma's? If I'd gone in and said no thank you, well, I can't even imagine the result.
Another, more relevant comparison: I grew up on a farm. My siblings and I did the work of making the farm run our whole lives. We drove trucks and tractors, hauled grain, chased cows, fed cows, harvested, seeded, etc. Sometimes, in harvest time, we even skipped school to finish the farm work. This work started at around 7-8 years old (younger for the more "domestic" tasks like feeding pets, cutting grass, clearing tables, etc). Now, a couple of my siblings are involved in farming and their children help out around the farm in age-appropriate way (though I think our definition of age-appropriate would shock some people). If I were to send my (imaginary) 13 year old to the farm for a few days (especially during harvest), you can bet that I know that kid is shovelling grain at the very least. And that's probably why I'd send him/her. It's not going to damage the kid to shovel some grain for a weekend in order to help out the family. Just like it wouldn't hurt him to hammer some nails or do some kitchen clean up.