Pre-1850 census records in the United States are notorious for only indicating the names of heads of households.
The 1845 Michigan State Census for St. Clair County, Michigan lists adjacent entries for Benjamin Butler and Anson Butler in Wales Township. Benjamin’s entry indicated the presence of one male under 10, one male over 21 and under 45, and a female over 21 and under 45. This makes it appear that Benjamin’s household is that of a fairly young couple. The household of Anson Butler has a male over 10 and under 21, one male over 21 and over 45, one male over 45, one female over 21 and under 45, and one female over 45.
The far right hand column of the census indicated who the males over 21 in the household were. For Benjamin that was just him. For Anson’s household it was Anson and a man who appears to be Abner Fitch.
One may speculate that Abner Fitch is a son-in-law of Anson Butler, but more work needs to be done in order to determine how reasonable that speculation is.