I am not a hygienist but my mother is and I am familiar with Apollo college because I live in AZ. Hygienists are not that in favor of colleges such as Apollo college b/c they will flood the market with hygienists lower the earning potential of hygienists. Dentists are in favor of Apollo college and other such programs for this very same reason.
About the education. It will be considered as substandard due to the fact the credits your sister earns will not be transferable to CC, JC, or University institutions. She will probably be able to skirt some pre-reqs that other hygienists needed to take. It is those extra credits/courses that make hygienists more than just technicians. Apollo takes what used to be bachelors degree and turns it into a 16 to 24 month program. Even those who earn associates in dental hygiene go to school for 3+ years.
Your sister will come out with a ton of debt. She will be paying out the nose for nontransferable credit from a school that current hygienists dont support only to drive down the earning potential of herself and future hygienists.
However, these schools probably are the wave of the future do to a market demand and the strong backing of dentists. The only way to stop them is to not enroll. She would be better off taking a longer route and going to a JC or University.