The Maggie B by Irene Haas. One of my favorite picture books ever. When Margaret sails away on a houseboat of her very own, she knows no adventure would be complete without her baby brother. They sail through sun and storm, and efficient Margaret whips up a sea stew so tantalizingly described you can almost smell it simmering on the page. This is a warm, merry book that celebrates the quiet joys I treasure, most especially that bond between the capable big sister and the baby she adores.
(More books with babies here.)