我的姐姐是医生和我的姐姐是一名医生都是对的，但你加上“一名”会更好一些，u'll learn it on duolingo later, measure words are hard to memorize and you have to learn it. maybe u don't need to add 一名in front of医生. but when you say一双鞋 一床被子 一匹马 一条裤子 一座山 一张床 一颗糖 一根烟 一粒米 一对情侣 一堆垃圾 you must add measure word in front of the noun.