Sister

sister (adjective)
belonging to a group of similar and related things, such as businesses, usually owned or operated by the same person or organization.

Example:
When could you introduce me to your sister company?
Ours and theirs are sister schools.
Note: We won't say brother company or brother school.

The word "umbrella" is sometimes used for similar purpose. Examples:
Mr. Hughes's agency is one of the community agencies under the umbrella of the National Community Action Foundation.
Donations should be sent to the Disaster Emergency Committee, an umbrella organization for UK based aid agencies.