I'm honest with my family that I smoke. You should only tell your mom if you're ready for your dad to know; married couples talk, even if they don't want to. The best thing to do if you're scared is give them reasons not to be concerned about it; try to be more responsible, help out around the place when you can.

Being secretive is easier, but being honest and having your parents know it doesn't effect your personality might even change their whole view on the herb itself. That's what happened with me.