Complex sentence: Schneider began participating in Seances with his elder brother Willi Schneider at age 11, at which he claimed to be channeling the spirit of Olga Lintner; a persona previously used by Willi.
Split into simpler sentences: Schneider began participating in seances with his elder brother Willi Schneider at age 11. He claimed to be channeling the spirit of Olga Lintner; a persona previously used by Willi.