My Family Tree

seems likely that Ellen, Margaret, and Thomas were siblings, however David Decker has Ellen born ~1802 to Richard Johnson and Eleanor Decker and married at 13 years old! Thomas Johnson was one of the earliest settlers in Knox Co, IN in 1800 and married Catherine Catt, same family as Deckers married. "History of Knox and Daviess Counties, Indiana". Johnson Township was organized in 1812 and named after Thomas Johnson.
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=dldecker&id=I04956

other family details from Michael Lamere family tree on ancestry.com

Samuel S Johnson was administrator of Jacob Decker's estate - seems likely he was brother of Margaret too. There was a Samuel in Knox early on:
Name: Samuel Johnson
Gender: M (Male)
State: Indiana
Locality: Indiana Territory
County: Knox County
Residence Year: 1810
Household Remarks: Name on petition, 1808, to President Jefferson by citizens of Knox County expressing their loss of confidence in John Rice Jones, a member of the Legislative Council, and recommending that the territory not be divided...
He died Sep. 20, 1845 in Bruceville, Knox, IN https://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=62541345
, but probate of 1830 mentions only 4 children not Margaret/Ellen, so not sure which Samuel Johnson that is.