My mum has a red blotchy rash all over her body, especially her legs. Some are small and some are. Big covering her elbows. What is it?

Red blotchy rash. It could be almost anything. I would have to know how long she's had it; what medication she may be taking; how old is she; and does it itch? From your description it could be a rash from a medication she has be taking; she may have a condition called psoriasis (happens often on the elbows); and without a photo there is no way of telling. A dermatologist would be your best bet for her.