without revealing too much... I'm in a similar situation...
If I were you I would have a talk with both mom and step-pop... if either one of them EVER go in the basement they will probably know. it is going to be near impossible to hide an active grow without spending lots of time and money on stealth.
better to bite the bullet now than to be half way done and your girls start to stink to high heaven and then your mom/step-dad finds em, flips her/his shit, and tosses the girls.
edit: also, not sure if you've scrapped the idea or not, but unless you happen to be a MASTER plumber I would not fuck with a water main. I know sweating a few joints is easy, but I've seen many plumbing jobs fall to shit because somebody thought they knew what they were doing, but didn't.