。
It's been ten years since my mother died.
Literal
Mother [subject-が] died and 10 years [into-に] becomes.
The pattern [verb]-て + [duration]になる is the idiomatic way to express 'X duration has passed since Y happened' — literally 'having Y-ed, [time] has come into being.' The verb stays in the non-past because なる continues into the present. Tight, compact, and very characteristic of how Japanese packages elapsed time.