What do you think they adopted a son for, Kaldin? They COULD have had another girl. His track record of avoiding farm work, his lack of interest in furthering his knowledge by going to ag college, is it's own evidence.
What? If they were desperate for a son, why didn't they adopt a baby boy instead of Sheila? Maybe they wanted a boy and girl, like people do, or maybe they just took a baby which was available. Girls can go into farming too, but there was no pressure on Sheila to do so.
None of that is evidence that he hated farming so much that he wanted to get out of it and run off with all the money - to the extent that he would murder five people, including two small boys.