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Amy Wainwright's two daughters, 6 and 3, were overjoyed to welcome their new baby brother, Gavin -- and he adored them right back. His first smile was to Cora, who was home with him when older sister Julia was at school. "When he got bigger, he'd clap and squeal when he saw her coming," says the West Roxbury, MA, mom.

Of course babies bond with their parents, but the attachment they build with older sibs can be superstrong, too -- and lots of fun. "Siblings are exciting to babies, since they move quickly and sound and look different than adults," says Maria Legerstee, Ph.D., of York University's Center for Research and Infancy, in Toronto. They're thrilling without being scary. Plus, they're naturals at knowing what to play. Cora enjoyed making goofy faces just to get Gavin to smile, and he'd gaze at her in total adoration.