At the age of 57 years, endometrial thickness of 5.4 mm is considered slightly more than what one would expect. It is reassuring to know that your mother does not have any bleeding.It is advisable to undergo hysteroscopy and D & C. This will help identify any suspicious areas in the cavity of uterus and take biopsies from endometrium (D & C). This will also help in ruling out any sinister causes such as cancer of the endometrium.It is very unlikely that there will be any cause identified but it is advisable to get the above procedure done and feel reassured.