How do I say “on her/his/their mother/father's side” in German?

Question

How do I say "She looks like her aunts on her mother's side"?

Sie sieht wie ihre Tanten mütterlicherseits aus.

I cannot use mütterlicherseits in this case because that would mean "on my mother's side". How do I say "on her mother's/father's side"?

Answer 1

Mütterlicherseits and väterlicherseits are absolutely correct.
You can use them in third person singular, as you did in your example.