Complex sentence: Yulii's mother, Mirra Yakovlevna Burovskaya, was a theatre actress who acted in various plays in the Moscow Art Theatre but left Russia in 1910 due to illness that had to be treated at the European resort.
Split into simpler sentences: Yulii's mother, Mirra Yakovlevna Burovskaya, was a theatre actress who performed at the Moscow Art Theatre. She left Russia in 1910 due to an illness that had to be treated at the European resort.