Complex sentence: Welwyn, the family mansion also designed by Delano & Aldrich, was built in 1913 at Glen Cove, Long Island, is now owned and operated as the Nassau County Museum.
Split into simpler sentences: Welwyn, the family mansion also designed by Delano & Aldrich, was built in 1913 at Glen Cove, Long Island. The property is now owned and operated as the Nassau County Museum.