False. But under certain conditions, it might be true.
Federal law has jurisdiction over marriage laws, and the Marriage (Prohibited Degrees) Act, S.C. 1990, c. 46 says that it is prohibited to marry our brother or sister, whether they are adopted or not (art. 2: «No person shall marry another person if they are related lineally, or as brother or sister or half-brother or half-sister, including by adoption»).
However, the Province of Quebec is also ruled by the Quebec Civil Code , and the province has the power to legislate adoption. At article 578, we can read that under certain circonstances, the court may authorize a marriage between an adopted person and a member of his or her family…. So general rule, no, but if you have really good arguments, maybe…!