I assume you are talking about the 1880? I found it long ago before digitized, before cd, back when all there was was microfilm and rarely was there an index.
It appears the indexer missed all three -- Peter, Elizabeth and John but the person did get Francis Hill the nephew. (Apparently I couldn't make his surname out years ago on the paper copy.) I have reported the missing people in the index problem to ancestry.
Anyways you can find them on image 5 of the Mussey Twp images. Here is the linkhttp://interactive.ancestry.com/6742/4241807-00090/32029286?...
That was a mess. If that link doesn't work. Either search for Francis Hill in Mussey or Search for with exact search Peter first name, Michigan, St Clair, Mussey and about 5 will come up. Select Peter Young and then click forward one image, Peter Kelly and family are listed below the Martin household. If you have problem finding them on other census years before this, remember this that the Young, Kelley/Kelly, Burris and McCoy families were all in the same neighborhood. Other surnames in the area at times were Cooley, Hodge,.