Due to some unexpected circumstances, I was unable to shadow another dentist during undergrad. However, over the years I had accumulated 150+ hours working and shadowing in my dad's office, and about 10 hours shadowing a pediatric dentist during high school. I had planned to do more, and I was very nervous that adcoms would question why I had done all my shadowing with my dad. It turns out that it wasn't a problem at all, they didn't even ask me about it -- some interviewers even commented that with several family members in the business, at least I knew what I was getting myself into! I would definitely suggest branching out and watching non-family members in action, but any experience is better than no experience.