Try and determine when your relative learned that story they are telling you. Would they have been a small child when they heard it? Memories that come from when the person was a child can be impacted by their immaturity and inexperience with life. Sometimes children draw interesting conclusions about family events only to pass them on as facts years later.
Just because someone heard or experienced something when they were young does not mean they could not remember it correctly. But it’s worth remembering ourselves that what children may not get details straight or make incorrect inferences.
Of course adults can as well.