As per KenM's comment, this might be the Young Wizards series by Diane Duane. The two primary characters are Kit and Nita, who aren't brother and sister, but are very close. They are in their teens, and they did gain their magical knowledge from a book that bears the same title as the first book in the series, So You Want to Be a Wizard, where they don't travel into space, but they do travel to a few parallel worlds, including one where it is always night, and inanimate objects like cars and fire hydrants are living predators that eat birds and mammals in the setting. The second book, Deep Wizardry, has them turning into whales to participate in a magical ritual. The third book, High Wizardry, follows Nita's younger sister, Dairine, who gains her powers from a magical computer. She travels through space.
Another possibility is that you might be thinking of Fred, the sentient white hole that accompanies Nita in the first book. He can indeed hold certain items in him, and can also disgorge other items on command.
The next day at school Fred and Nita attempt to retrieve Nita's pen from her bullies, but Fred miscalculates the gravity required to lift the pen from the bullies' possession, and instead accidentally swallows it. This makes Fred hiccup objects, including a television, a blue Mercedes, and a LearJet. After school, they seek help from the local Advisory Wizards. Fred's problem is fixed, but Nita's pen is not recovered. In order to retrieve it, they must link Fred to a worldgate in Grand Central Station and pull the pen out of Fred.
The two big overarching themes of the series are a) magic is a matter of understanding the language that defines the world, and changing a thing's name to change its nature and b) magic is used to fight the Dark Power, which introduced death across the universe, and exists in many times and dimensions, which means he shows up and must be defeated again in almost every book of the series.