When people want me to work on their car, my rates vary. I am always willing to teach someone how to repair/service their vehicle. I usually don't charge for that, unless they volunteer to supply beer.
As far as doing the work, I charge something like $15 per hour because I am not a certified mechanic. As a result of that, I tell them that I have a 3-foot/3-second warranty on all my work. I keep all the old/bad parts for their full inspection. I supply them with a detailed receipt, which I make them sign, showing work done and noting my warranty mentioned above. I also will not do any work for known whiners (like half of my in-laws).
Family is a lot easier if they know you can be an uncouth, grouchy SOB. There are family members I've told to pound sand (in words not so kind) when they wanted me to work on their car, or anything else. I've told my sister that there are plenty of mechanics in the yellow pages.
I long ago determined that, family or not, if they don't care about my feelings, I'm not going to spare theirs.