OK, I'm typing this as I'm working on your question.So, I'm at Ancestry.com.You've given me that Margaret Harrison was born 1850 and, of course, sheprobably has the same father, James.
The 1st thing I find searching for Margaret, is a posted family tree for Margaret, born July 1850 - in Ireland - no parents listed, so no sister listed.But, this says she married William K. Austin about 1878 and that she died in Brooklyn, May 5, 1916.It also gives her children.
Doing a quick perusal of the 1850 census listings that come up, I'm not finding Margaret with James but I'm doing it quickly.It may be that they arrived that year and were missed by the census.So, I'm going to look at the 1860 census when Margaret would have been 10 and maybe Agnes would have been born by then.I'm not seeing anything that looks right so I'm going on to 1870, 8 years before Margaret married William.Maybe they hadn't actually come to the US yet.All I've found is a 25 yr old Margaret by herself living in the 15th Ward NY, wherever that is.
I'm going to go on to try to find James or Agnes.Again, I'm doing this quickly just to see if I can find anything.Nothing so far.
So, I'm trying a couple of different things and in the family trees on Ancestry.com, I find a William H. Wormuth married to an Agnes R. no last name and a daughter Lucy born 1877, so maybe not.
Do you know if she had children?Do you know when or where she died?What is your connection to her, that might give a clue?Margaret's children might know who Agnes married.