The term "sister company" has no legal significance whatsoever, and is not a term of art in law, finance, or accounting. It is purely a marketing term, it is meant to imply a close working relationship and some sort of endorsement. Sometimes companies with common ownership or management are called sisters. Typically, companies with a real legal or contractual relationship will describe that relationship using a more accurate and significant term, like "vendor" or "subsidiary" or "strategic partner" or "licensee.