The first step for anyone seeking a sibling or child placed for adoption in Ontario is the website Access to Adoption Records Act maintained by the government. From there you can apply to the Adoption Disclosure Registry. This registry exists to bring together adoptees and birth relations and is free to use.
While awaiting a match, if the adoptee was placed through the Children's Aid Society you can apply for non-identifying information. There are also numerous internet sites set up for this type of search where you can post your information and hope the adoptee sees it and gets in touch. Be sure to also seek out local adoption groups (such as Parent Finders) for advice and support.