Hi Chelsea - that is very interesting. Do you know how Stephen Cook ended up from Ontario to Indiana/South Dakota? I have found Lydia Cook died in Wisconsin and her sons Joel and James lived in Wisconsin for some time (Bascobel, Wisconsin). Unfortunately, while the Federal mortality census is pretty clear about Lydia and her ties to her son, Joel, the census taker listed her birth place as Massachusetts (which was her father's Nathaniel's birthplace).
It looks like most of the Lydia Churchill/Daniel Cook family left Ontario and settled in the United States sometime in the 1840's to 1850's. I believe James Cook (the youngest son) was the first to leave Canada for the US. I also believe Lydia Cook (their mother) may have left Ontario upon the death of her son Isaac in 1858. It appears all of the other Cook boys left for America except Isaac, who was working the farm in East Zorra.