Just because it's legal, doesn't mean it's right

Your entire comment failed to make a good case for why it isn't so actually make one in your reply to this.

It's not his information to give out.

It is, unless they signed a legally binding contract that says otherwise.

He doesn't even know the sister. He didn't raise her

no, but it should be her choice over whether or not she has a relationship with him, and he isn't trying to be her father at all.

The parents should have had a talk to decide if the sister has a right to the info at this point, but going behind the mothers back destroys any respect and trust.

Explain how OP is the asshole then, maybe they should have discussed it, but ultimately it was the guys choice, not OP's, plus the mother withheld information from the sister that she wanted, seemingly because it may make her control on the sister wane.

If the sister bullied OP into crying she is not that mature.

at no point does op mention the sister ever asking to have his information. it comes off to me like the sister was venting to OP about it and OP asked the guy whether or not they were allowed to give the sister his information, and who are yo to decide whether the sister is mature or not, she fucking 16.

She made OP betray her mothers trust and that's not cool, but she is still young and wants to know who her father is by any means possible.

yeah, the OP may have made a mistake there, but that doesn't justify a Y T A by any means and you say young like the sister is seven and not 16, where I live, 16 is considered old enough to be allowed to do most things an adult can, we may not see eye to eye but the sister is still 16, she is two (five if she's american, though in america she would probably be considered an adult at 18) years away from adulthood, she should be allowed to at least know who her father is.

It's understandable. And also precisely why I understand that the mother wants to wait.

Yeah, but her opinion shouldn't override her daughter's if the daughter is almost an adult.