If someone contacted me now and told me that was the case, that my 16 year old sister was really my mother this is what would happen:
I would hate the person that told me and ruined the relationship with my family.
I would hate my "parents" for the lie.
I would hate my "siblings" or who I thought were my siblings for the lie.
Basically I think I would end up losing contact an hating every member of my family older then me. I honestly don't see me feeling any other way then honestly cutting off most of my current family moving away with my husband and starting over without any of them becuase I would be so upset about letting me live all my life as a lie.
So as much as I generally like truth in this instance I would have prefered that no one brought it up and feel like by telling me they ruined everything. So it would make me go from having a loving family to no family at all, all because some selfish person couldn't keep his mouth shut and just let me keep happily living the lie.
I wanted to respond to this part because I have a relative who adopted his daughter when she was 2. He has known her since she was born and raised her as his own since the day she was born. The girl is closer to my grandparents then my whole family(parents, sister's and their families) We have always lived far away from our family and my grandparents watched her when her parents were working as a child. The girl is now in her 30s. She only found out she was adopted in her early 20s. We were at my grandfather's funeral and she happened to mention that she had discovered her birth certificate was changed. We don't know what she was told but my relative was there since her birth and married her mom when the child was 2 so she is in the wedding pictures. When she was younger I know they were planning to tell her she was adopted but she was watching the movie Annie and remarked," I'm glad I'm not like that girl I know who my daddy is. Ugh!!
The part that gets me in this comment regarding how this person would feel knowing others in the family knew and didn't say anything. I have tried to put myself in her situation wondering if she would be upset at everyone in the family for not saying anything. Would she be angry that I knew? I was 10 when she was born and I just accepted her as my cousin. It isn't my place to say anything. I guess if it is ever found out I could just act like I didn't know what happened. Heck I only see the family twice a year growing up and haven't been back since 2009. I guess I just feel some guilt because of keeping the secret. Honestly I can't imagine our family without this girl. Her parents are now divorced and both have a great relationship with their daughter.
Just to offer a different perspective, on what it feels like to have family secrets suddenly come to light...
I have a friend whose mother told her that she was her "one and only". Her mother told her, repeatedly, that my friend was the only child she'd ever had.
Fast forward many years, and my friend gets an unexpected invite to a "family reunion". She's thinking, "How much family could we possibly have?" but goes, expecting to meet perhaps some unknown cousins.
Instead she meets three half-siblings.
It turns out her mother had a number of other children with previous partners, whom she'd given up for adoption, and they'd all finally tracked each other down. Her mother's only comment was, "I didn't think it was any of your business." My friend is now convinced she must have spent a little bit of time in foster care as a toddler (though her mother denied it and swore the vague memories she had of living with a nice older couple were just of her "babysitters").
She didn't hate her mother for the lie. She thinks her siblings are fantastic and is thrilled to see how well they've all done for themselves. They all stay in touch on Facebook.
She does wish her mother could have told her more of the truth before she died. All of the siblings wonder if there aren't more of them out there somewhere. But, if anything, having even just a bit of the truth finally come out helped my friend understand her mother better and make peace with her before she died. She's even more tolerant of her mother's widower, than she used to be.
I can definitely see being upset about being lied to. My friend was definitely upset! But, at the end of the day, my friend still had a mother (and some new siblings!).
So... I guess my question for you is, would you
really hate every member of your family older than you and end up with no family at all, if someone told you they were your biological parent? Is biology the only thing holding your family together right now?
(That said, your cousin already knows she's adopted. I really don't think it matters anymore what you knew when you were ten.)