Remember that family members can easily individuals from previous generations confused creating additional confusion for the researcher.
An ancestor's wife's name was Ellen. His sister was Emma. The more I learn about Emma, the more I realize that some of the stories that were told about Ellen were actually about Emma. It is easy to see how one could get the names mixed up, particularly if one had never met either person.
Sometimes the mix up happens when the names are not similar at all. Is it possible what grandma told you about relative A was actually about relative B?