Question: are gaara temari and kankuro really siblings?
Answer: Yes, it's been established even in Part I that those three are siblings, when Kankuro remarks that even though Garra's his little brother, he's still scared of him. In Shippuden, they're also remarked as the Sand Siblings; Temari's the oldest of the three, Kankuro being second-oldest, and Garra's the youngest.