I don't have any further info on Lucy M. other than she was alive and listed on the 1880 census living with her other younger siblings as Wards.I noticed on an Indiana site at USGENWEB that you said she was the one that died at age 4.
I am not researching the family myself.I am just a lookup volunteer.
The 1850 census that lists Nathan does not have him living with his parents.But looking at other census I think it's a GOOD chance that his parents were the people that I mentioned previously.
1850 Montgomery Co., IN -Union Nathan Graham -age 23-born Indiana--clerk for William H. Boynton (merchant)
next page is Noble B. Graham age 53 b.KY. He is married to his 2nd wife and has some children at home.
same county and township is: Joseph H. Graham-46-KY Sarah-42-KY John T.-21-IN Margery E.-19-IN Fergis F.-18-IN (male) James W.-15-IN Margaret A.-13-IN Wm H.-10-IN Mary J.-8-IN Martha E.-6-IN Emily F.-3-IN Joseph T.-1-IN
Earlier census for Joseph Graham does have an age appropriate male child that could be Nathan.Joseph and Sarah's marriage date is a good time for them to be the parents of Nathan. Noble Graham did not have an age appropriate male child in earlier census.