“Happy Birthday” may be the Hill sisters' main claim to fame, but Patty (left) and Mildred's (right) impact on American history extends far beyond the beloved song. Illustration by Meilan Solly / ...

It’s no secret that the bond shared between sisters is special. She’s the one who knows all your secrets, knows what makes you tick and will do just about anything for you. If you can’t lean on your ...