Complex sentence: When their sister Helen was abducted by the legendary Greek king Theseus, they invaded his kingdom of Attica to rescue her, abducting Theseus' mother Aethra in revenge and carrying her off to Sparta while setting his rival, Menestheus, on the throne of Athens.
Split into simpler sentences: When their sister Helen was abducted by the legendary Greek king Theseus, the brothers invaded his kingdom of Attica to rescue her. In revenge they abducted Theseus' mother Aethra and took her to Sparta while setting his rival, Menestheus, on the throne of Athens.