An inlaw's mother was raised by a maiden aunt. The whole family ie: Grandparents, siblings etc all immigrated to New Zealand when she was 5. Aged 14, the woman she called Mother aka Aunt died and she then was made aware that the woman she called Aunt was her mother.... I think everyone knew but her. At birth she was registered properly and would have to know at some time but it was an awful shock to her the way she found out.
rachelralph.....I wonder if there are any living people who you could ask about the family or maybe neighbours etc that would tell you more candidly about this. So many folk that are unable to conceive maybe it was like that. I think so many things that are innocently done turn out to be a real dog's breakfast. One tale begets another and so on.
Good luck. Keep us posted.