I'm not a professional. I work mostly on my own cars. I have done full details on Mom's, Sister's, and Brother's cars. I didn't charge them a cent.
See, it doesn't matter who someone is, or how they are related to you, when they give you money, they expect something in return. And they expect that something to be commensurate with the amount of money that they paid.
Even at half off, people are still expecting their cars clean. What if you left a scratch? What if you didn't get that stain out of the carpet? What if the windshield had streaks? Now that you've been paid, you're obligated (at least morally I think) to provide quality service. And if you make a mistake, or if someone is super-picky, or if they were expecting something else, then you're stuck. You get caught in this pickle where you either do more work than you're getting paid for, or you've got a family member pissed off at you.
Lesson here: DO NOT CHARGE FAMILY MEMBERS FOR DETAILS
I don't charge my family for details, but I do return favors. My mom's car got done because she babysat my daughter while I was on a business trip. My brother's car got detailed because he helped me tile my kitchen floor.