I Otis & Clara In 1870 in Walworth County, Wisconsin
In 1880 in Jo Daviess County, Illinois
In 1900 in Grant County, Wisconsin
In 1910 in Hardin County, Iowa
In 1920 in Cerro Gordo County, Iowa (Clara)
So, this information suggests that your Dempster Thomas was born in Walworth
County, Wisconsin (9-15-1870).
I found Otis with his family in 1860 in Walworth County, Wisconsin.
If you haven't gotten birth, marriage records or death records of all family
members, I would suggest doing so on the ones attainable. Go ahead and pay
the cost for all family members if it means you might get an answer
Okay, with the info that in 1860 Otis lived in Walworth County and he
supposedly married Clara in 1868 ant they still lived there in 1870, I would
suggest that is where they got married and center my search around this
Look for any Sackett families in the surrounding counties in 1870. Because
Walworth is a border county, also check Illinois. Even though Clara won't
show up in this year census with her "birth" family, you can follow the
family names back into 1860 & 1850 and see if she is with any of them then.
What religion was Rev. Davis? Possibly look for a church in Walworth
County, Wisconsin that was around in 1868 and write them to see if they have
any records. Of course, a small donation would be appropriate to have them
look for you.
There are quite a few entries on this family on ancestry's world tree. If
you don't have these, let me know and I will extract them for you.
Good luck and I hope I gave you some different ideas.