37 here.
I mean it could become a problem later if she finds out about it. She'd most likely feel very betrayed by both you and her sister. Finding out that you hadn't been honest with her about something so sigfnificant, finding out that the man she'd been hoping to marry had been hiding such strong feelings for her own sister... would be extremely upsetting to most women. And to find out that her own sister had kept such vital information from her would be absolutely horrible. And it's likely that her sister would want to tell her, straight away or even many years later- most people don't like to keep important secrets from their family. If I was the sister and you told me that, i would tell her right away, because family comes first and i would feel wrong about keeping it from her. I've asked a couple of my friends and my mother, and they all say they'd tell her straight away, too.