Depending upon the mother, brother, or girl being the object, 'who' might be 'whom'.
I have some opinions on that based on my interpretation of CMOS. I cannot guarantee I understand what they mean correctly.
The antecedent of 'who' is definitely the mother.
5.57 Antecedent of relative pronouns - Usually a relative pronoun's antecedent is a noun or pronoun in the independent clause on which the relative clause depends. For clarity it should immediately precede the pronoun
The case of the relative pronoun, 'who' or 'whom', depends on whether the pronouns antecedent is the subject or object of the following verb. If 'whom' is needed there must be a subject between it and that verb. For example, I think this is technically correct: "... ran to their mother whom they clutched in their arms." It may be better to rewrite a sentence than use 'whom' correctly.
5.56 Positional nuances with relative pronouns - A relative pronoun is in the nominative case when no subject comes between it and the verb
the professor who lectured was brilliant
he whom I called is no longer there
5.55 Case with relative pronoun - A personal pronoun does not govern the case of a relative pronoun. Hence, an objective pronoun such as me may be the antecedent of of the nominative pronoun who.
She was referring to me, who never graduated from college.
A construction formed this way sounds increasingly archaic or (to the nonliterary) incorrect ... the best course may be ... to find a different construction.