Here's why:
* Pronoun Agreement: Pronouns should agree in number and person with the noun or pronoun they refer to. In this case, "your mother" is singular, and "he" is also singular.
* Parallel Structure: It's generally considered good grammar to maintain parallel structure when listing items. In this case, "your mother" is a noun phrase, and "he" is a pronoun, but they both function as the subject of the sentence.
Example: "Your mother and he are going to the store."