Thanks again. I'll try to help with some Census data, courtesy Cheryl Helm.These are two brothers, matching the sons of the Andrew mentioned in the LDS source. Spouses, and children of Casper, match too. Their sister Anna Elizabeth is likewise well known.
1870 US Census
Fayette County, IL, Wheatland Township, Hickory Creek PO
Youst Yakel, age 40, born Prussia Catherine, age 34, born New York
Next door is
Casper Yakel, age 43, b Prussia Hannah, age 37, born Prussia Liddia, age 11, b IL Elizabeth, age 7, b IL