Here's why:
* Mother is a noun that refers to a female parent.
* She is the pronoun used to refer to a female person in the third person singular.
* Her is the possessive pronoun used to show ownership or belonging to a female person.
However, it's important to remember that not everyone who is a mother identifies as female. Some mothers may identify as non-binary or transgender. Always use the pronouns that a person asks you to use for them.