Complex sentence: Her younger (by 19 months) sister is singer-songwriter Amanda Sudano of Johnnyswim, and an older half-sister, Mimi Sommer from her mother's first marriage to Helmut Sommer.
Split into simpler sentences: Her younger (by 19 months) sister is singer-songwriter Amanda Sudano of Johnnyswim. She also has an older half-sister, Mimi Sommer from her mother's first marriage to Helmut Sommer.