When your "sister" is your mother????

I need some seasoned researchers on this one...

I have a cousin that was born approx. 1892 in OH, raised as a sister...but was actually the oldest daughter's child....

I have a copy of the 1900 census that lists everyone in this household except the cousin.

Do not find her listed with any nearby relatives..searched on ancestry with surname and approx. age in that county and nothing came close to it...

My next thought was to look for the maternal ggrandparents.

A family "rumor" is that she was fathered by the employer of her mother/sister, who was listed as a servant on the census..don't have a clue who she worked for...

WHAT DO I DO????