Which prophetess was judging Israel when Sisera's army oppressed them?

Answer: Deborah

Deborah, a prophetess and the wife of Lappidoth, judged Israel and summoned Barak to fight against Sisera.

“Now Deborah, a prophetess, the wife of Lappidoth, was judging Israel at that time.”— Judges 4:4 (BSB)