Here's why:
* Many Pitbulls are adopted: A lot of Pitbulls come from shelters or rescues, and their lineage may be unknown.
* No central database: There isn't a global database tracking every Pitbull's parentage.
* Breed is not a guarantee: "Pitbull" is a general term that refers to several breeds, and there's no official "Pitbull" breed recognized by major kennel clubs.
If you're interested in learning about a specific Pitbull's siblings, you'd need to reach out to the breeder, shelter, or rescue organization where they came from.