Up to 1952 it was a legal requirement to notify all changes of name or address, but after National Registration ended it was a matter for the NHS. If a woman didn't notify her doctor of her change of name, or the doctor didn't pass the information, no-one would be prosecuted as a result. I have one case where a woman married in 1944, and again in 1958, but only the 1944 married name appears in the Register. If your mother married after 1952, and her sisters married earlier, that might be the answer. If not, bang goes my theory!