Rachael, I'm angsting over a similar problem at the moment, and I agree with all of the other comments and answers previously posted. In my own case I've collected over 75 official documents and STILL cannot "prove" the parent! I'm close, but not close enough.
Here are some things I've searched: US Census, including whole neighborhoods; birth, marriage, and death records for parents, grandparents, siblings, and children of siblings; obituaries; newspaper articles; other web-site genealogies (untrustworthy, but helpful); and many other sources that may not be applicable in your case, including church records.
It was helpful to produce a "timeline" for this family. In the timeline I used color-coding to help me sort out sources, names, dates and the like.
Hopefully the answer will come to you soon. Even if it doesn't, posting your research might help others down the road.