Complex sentence: In the harsh world known as the Age of Apocalypse, the St.Croix twins were called "The Monets" and worked alongside their brother Emplate in Apocalypse's Elite Mutant Force under the supervisation of Mr. Sinister.
Split into simpler sentences: In the harsh world known as the Age of Apocalypse, the St.Croix twins fall to their brother's control and alongside Emplate they've joined Apocalypse's Elite Mutant Force. The twins were called "The Monets" and worked under the supervisation of Mr. Sinister.