I know that the Donner party incident happened earlier but Is there any record of a relative named "Sarah Donner" going to Oregon soon after? I found this book With many other old books.
At least in my state, a lot of the census records have been digitized (and are therefore searchable) at universities and public libraries.
Even simpler, you could email the history department at Wake Forest University, which is in Salem, NC, where the Donners are from. I imagine they would know or could quickly find out, because this could be quite an interesting find.
It looks like the Donner family archives are now curated by the library at the University of Illinois at Springfield (UIS). The materials they currently have are listed here, with a contact link on the bottom left. You might want to check if anyone who participated in the acquisition (fairly recent: in 2008) also charted the Donner family tree, and if so, whether there was a Sarah Donner who also migrated after the ill-fated party of the same name.