I've had my daughter work with me a bit. She sometimes did great, other times had to go sit in the truck, depending on her moods. My other daughter isn't as handy but follows directions better. She's never helped on a job but does OK when helping around my shed, cleaning up, etc.
All depends on the individual. A solid person will make extra effort to not embarrass the family member. A lazy person won't care & will milk the relationship for all it is worth.
I worked at a firm where a partner was a relative. I never slacked off, did my best work, as I always do. If I did screw up (yes, sometimes), I got scorched by him at least as much as anyone else would have. I would also keep an extra low profile until things settled down. Not show my face if I didn't have to, arrive early & start work, avoid the morning gatherings, etc. when I could. Coworkers knew I had no special influence or knowledge. I worked for him twice. When work slowed down I went elsewhere. I still help out occasionally if they need an extra man for a day or week, etc. I have "borrowed" a guy from them too.