My (19M) sister (17F) is adopted, I am not. They decided it was best to wait until she was ten to tell her. I didn't really care, she was and still is my sister no matter what. For some reason though it really set her off. She accused them of "stealing" her, and overall was nasty for a few years.
We had to put up with a lot of "You're not my real dad/mom" bullshit. When she was like 13 she accused them of being human traffickers for a while. And generally she was just awful and ungrateful. She kept badgering them to let her meet her bio "parents" but they refused, since they gave up the right to have any relationship with her. Myself ad her had a lot of fights about it to, sometimes physical, I just hated her for what she did to our parents.
The issue is that my sister will be 18 in a few months, and she'll be able to get all the info to actually find them. I don't get why she's so obsessed with meeting people who clearly never loved her. Our parents actually put in the work to raise her, she should be giving them her love, not some punks who pawned her off as soon as she was born.
So I gave her an ultimateum. Either she drops this now, and pretends like she wants nothing to do with her bio "parents" (fake it till you make it) or I'll be out of her life, after all I'm not her "real" brother. I also told her since she wants to meet her bio family so bad, I'll let my family know and make sure they never speak to her again either. She's been bratty and agressive woth everyone, and I'm concerned I might be taking this all too personally.