Shouldn't "The mother of a mother is a maternal grandmother" also be an accepted answer? I'll report it just in case it is.
That is correct. "til" is used to mark possession much like "of" in English.
While I think it's ok to write that in norwegian and english, it's not a direct translation. That would be "en mor til en mor er en mormor" or "en mors mor er en mormor"