This is the centre of the quilt. The large block in the centre is Andrew and Alice Criswell, Mary and Agnes's parents. E3 is Mary Criswell's wreath which is the same as D6, Martha Lambourn's. (The pattern is the same but not the date - Mary's is dated March 13th 1852 and Martha's is 4th Month 2nd Day 1852. Mary was a Presbyterian and Martha a Quaker.)
Mary's sister Margaret is F4 and I'm not surprised to find that the 'new' sister Agnes is next door F5. There are other important blocks here too.
C3 William Criswell, one of Mary's brothers (deceased)
C4 Alice McClellan, Mary's Godmother
C5 Lizzie Lambourn, Martha's younger sister
C6 J Dickey Smith, Agnes Criswell Smith's son and Mary's nephew
E6 Martha Richmond, Alice McClellan's niece
One of the unanswered questions is as follows:
Theory D: All of the above, but I will never know for sure. If we had all the answers then there would be nothing left to wonder about.