。
Mother is the earliest riser in our family.
Literal
Mother [subject-が] family [in-では] most early gets-up.
一番 ('number one, most') is the everyday superlative marker — more common in speech than もっとも. The composite particle では topicalizes the scope of comparison ('within the family'), marking the set within which mother is ranked. 早く is the adverbial form of 早い.