What causes Art to feel a kind of rivalry with his long dead brother, Richieu?

Richieu is Vladek and Anja's first child, born in Poland in 1937. In 1943, Vladek and Anja send him to live under the protection of Uncle Persis, where they think he will be safer. Richieu travels with Anja's sister, Tosha; Tosha's daughter, Bibbi; and Vladek's niece, Lonia. But soon after, Zawiercie is liquidated by the Nazis. Rather than be taken to the gas chamber, Tosha poisons herself and the children under her care, including Richieu. After his death, Vladek and Anja keep a photograph of their first child hanging on the wall of their bedroom, and Art comes to feel a sense of sibling rivalry with his "ghost brother."