Parents and brother on weed? Be thankful they're not on something like coke, heroin or meth.
Gateway drug, my ass. Only stupid people move on to more harmful drugs. And the more stupid people that do themselves in with heroin, the better our gene pool will be in the long run. ;-)
*Anything* is bad when used to excess. You can blow a ton of cash on marijuana just like you can on an online multiplayer video game. In other words, balance. If a person is holding things together, job, family, bills, etc., and likes to smoke some weed, what's the problem? If a person is blowing money on weed (or any recreational activity for that matter) that they should be spending on important things, and letting their lives go to shit, then yeah, there's a problem.
And of course, that's all you hear about, is the bad cases, hence the fear instilled in you. If things are going alright in your household, then let it go. Getting the authorities involved will be more trouble than you can possibly imagine. It's unfortunate that your parents haven't pulled you aside and had a talk with you about marijuana already, but regardless, just let it go. Otherwise, you're just another sad victim of the american "selective puritanism" machine.