If your sister has an exemption certificate either due to other medical conditions or low income and she has been diagnosed as Coeliac and not intolerance then yes she can get some bread or pasta and flour. Having Coeliac disease does not qualify for an exemption certificate though. This doesn't stop her having the goods prescribed and paying the prescription cost if diagnosed.
Gluten free cakes are easy enough to make with ground almonds and a lot cheaper than shop bought stuff