What error does this sentence contain? Which was her mothers favorite vase in all the world

The way "mothers" was used in this sentence was a plural for mother, which is grammatically incorrect. The sentence is meant to say that the vase, was the mother's favorite, as in possessive, not plural. Mother is missing an apostrophe between the r and the s in mothers

so: mother's

also there should be a question mark at the end of the sentence