There's no doubt that a baby knows it's mother from day one. But what about those other people running around the household? For instance, when does a baby recognize its siblings? Your infant might know his brothers and sisters sooner than expected.
In fact, babies are sort of hardwired to pick up on facial recognition. As noted in What To Expect, babies can recognize faces before any other pattern or object. Thanks to this innate power, your newborn will recognize faces he sees daily by the time he's only a few weeks old, as noted in Baby Center. If those siblings live in the same household, then chances are he'll know them in no time.
So if you're in the delicate position of integrating a new baby into the family, you can use this sibling recognition as a benefit. In fact, research reported in Popular Science suggests that babies who have siblings tend to have an easier time with facial recognition when they're older. So you can reassure your older child that she's helping the baby learn and grow, simply by looking at him.
Another way to help new siblings bond: encourage your child to make faces at the baby. As noted in What Not To Expect, even infants may try to mimic these exaggerated smiles and other funny expressions. Your youngest child will get a chance to study his favorite subject (a face), and your older child will probably crack up trying to make the most ridiculous expression for her sibling. It's a fun way to kick off their lifelong bond. Oh, and keep your camera handy for those instances that get extra-silly.
If your baby's siblings live elsewhere, or you have an only child, take heart: you can still encourage your little one's natural desire to study faces. For instance, you can get a book that shows off photographs of various expressions. Baby Faces by DK Publishing is pretty much a classic in this arena. And, for an adorable, personalized take on this idea, you can make a board book featuring the faces of your baby's immediate family members. Hopefully, by the time your little one sees these people face-to-face, he'll already recognize them.