Hi, every dietetics course is very highly competitive and requires an interview before you get offered a place, so it does not come up in clearing, as if spaces did come available those on the waiting list after having an interview would then be offered places instead. With her current grades aswell it would be unlikely through the traditional route she would be offered a place with her grades being CCC.
Your sister has two options: 1) Resit her A-levels and apply through traditional means to do Dietetics next year
or option no.2 which is what I recommend
2) Your sisters best bet is to go through clearing/apply now if she has time to an undergraduate course in Nutrition and then apply to become a Dietitian in her final year of university. Universities such as Leeds Beckett, London Metropolitan and many other unis would probably have spaces for her to undergo this degree, either through clearing or through traditional means.
Nutrition 3 years UG and 2 year MSc post grad in Dietetics, and you can become a dietitian by the end of it. This would only be one year longer than resitting your A-levels and you get a Master's Degree out of it.
Any other questions let me know.