The main reason you shouldn't work with your family, specially your parents, is that there is no line between your professional relationship, what they know of you, and your private life. If you slack at home, for them it means you'll do the same in your work. As soon as you show signs of being displeased, you are seen as the slacker you are to them out of work, and they start saying things about work at home and vice-versa. Which you find unnerving, and mostly unfair because work relations are not meant to mingle with personal ones for you. And for them they just want to work you until your motivate. Although they don't know that's really not how you motivate people.
And, of course, this leads to more fights about the subject, and you both end up regreting the decision and possibly resenting your parent for treating you differently from others.
Of course this may not happen, and you might work well with them. It depends on the kind of people you are, and if you do looooooooove that job.
