After his parents died, he was raised by his aunt.
Literal
Parents [possessive-の] after-death, he [topic-は] aunt [by-に] was-raised.
死後 (after death) is a compact compound: 死 (death) + 後 (after). 育てられた is the passive of 育てる (to raise) — he was raised by someone else. 叔母 (aunt, specifically parent's younger sister) uses に to mark the agent in a passive construction.