Complex sentence: The first member of the family who was known to use the name "Rothschild" but the family name is actually Bauer... was Izaak Elchanan Bauer, born in 1577.
Split into simpler sentences: The first member of the family who was known to use the name "Rothschild" but the families legal birth name was actually Bauer. It was Izaak Elchanan Bauer, born in 1577.