This is an expression that people use when they're shocked or surprised. This can be for positive reasons, like when someone says that they're pregnant, or for negative reasons like when someone has gotten in an accident.

When someone tells you good news, you might expect to say "congratulations". This would be the polite way to react if a coworker or not-very-close friend said that she was going to have a baby. But with someone really close like your sister, "Congratulations" is too distant. So a more excited response like "Oh my god!" is better.