Annie,
I don't know about a step brother named William, but it does raise the question. Could she be named after her father named William? Is it possible that he grew up around Bonar and possibly ran away or died somehow? If he is from the area would there be records of him, birth or death, some sort or roll?
I will ask my son-in-law about this, to see if his mother remembers anything about her Grandmother. If he comes up with something I'll post it.
Again thanks for your help, it is appreciated.
Chip