she is nurse and mother all in one to the orphans.請問這句的nurse和mother前為什麽不加不定冠詞a?

a is an indefinite article used before countable or singular nouns referring to people that has not already been mentioned
any; every is used to show that somebody is a member of a group or profession(=nurse)eg:-She's a Nurse.--She's a mother.
singular is a form of a noun refers to one person. The verb should be in the singular done by "is" already.
Compare all in one; all others are in plural,having the singular form idiomatically.

這裡的名詞nurse/mother指的是抽象的性質，而非具體的實物，類似不可數的物質名詞。
如：
The bridge is made of stone.（物質名詞）
He threw a stone at the dog. （普通名詞）

all-in-one:(adj.) combing two or more items or functions in a single unit
an all-in-one shampoo/conditioner
片語放在名詞前當「修飾語」時，字間加hypeh；但放在be動詞後，當「主詞補語」時，不加hyphen。所以你看到了all in one在本句。

nurse and mother與後面的代名詞all為「同位語」關係。