I live with my mother, younger brother, and grandparents.
Literal
Mother, younger-brother, grandparents [with-と] together live (polite).
The と一緒に ('together with') construction marks a group with whom an action is done. Note how Japanese lists the family members with just commas and a final と — similar to how English uses serial commas plus 'and.' 祖父母 is a compound for 'grandparents' (both grandpa and grandma at once).