I have another one - can't believe I didn't think to post this one first:
My aunt is my real mother, but I only found out when I was in my late teens.
She had me at 14, and it was all kept very hush-hush ... my grandparents were livid, and sent her away for a year for "study" (that's what they told everyone, but really she was sent to a farm in the middle of nowhere to continue her schooling from home and have me). When she returned they pretty much shunned her.
The woman who raised me, who is really my aunt and is 14 years older than my real mother, was always super-strict with me compared to my older sister, but I had an amazing childhood and love all my family deeply.
I used to go and stay with my aunt (real mother) at school holidays to help her with her kids, and really loved it ... I felt like I could tell her anything and she was more my friend than my aunt ... we're also very similar in looks (not unusual for aunts and nieces though in my experience), and have the same taste in a lot of things. When I found out that she was my real mum it was a nice feeling but nothing has really changed. My aunt who raised me as her own was scared I'd turn against her, but no ... not in a million years.
It was different times back then and my grandparents were very strict ... I think a lot of emotional turmoil was experienced on all sides which still runs deep today (even though they've long passed).