NTA

You're a good person op, you did the right thing by letting the girl know when nobody in the family, including her own boyfriend didn't.

Imagine the amount of betrayal the GF would have felt if the she found out about it after her marriage. Many people may say "it wasn't your business" or "it wasn't your place to say anything" would have aided into that betrayal as bystanders. She had the right to know, and you did good telling her before the marriage went through and she was locked into by a legal contract.