Complex sentence: She learns that the Meru and Maru have no recollections of who or what they are, but through a children's lullaby that Meru sing, Natsumi deducts that they must have family, especially parents somewhere.
Split into simpler sentences: She learns that the Meru and Maru have no recollections of who or what they are. When Natsumi hears a children's lullaby that Meru sing, she deducts that they must have family, especially parents somewhere.