Listen there's 2 ways this can go:
She'll break up. Ok you're only 21 and this isn't a fairytale but at least you did the honorable thing of telling her and somewhere down the line, she'll be thankful to have known that
She won't dump you. She might not confront her parents about this or she might but then it's none of your problem. You aren't at fault as you were just banging a chick at the gym who desired you. You're at fault for banging her daughter and keeping this a secret from her and this is something that can be used to blackmail you in the future.
Get your shit together and be a fucking man of honor. Don't try to keep this a secret from the girl you love, if you really love this girl.