。
Mother is always complaining at me.
Literal
Mother [topic-は] always me [to-に] complaint [obj-を] is-saying (polite).
文句を言う literally 'say complaint(s)' is the standard idiom for 'to complain, to grumble.' Paired with いつも~ている, it describes habitual repetitive complaining. に marks the target (the person complained at), not a location.