The term "at arm's length" describes a relationship where persons act independently of each other or who are not related. The term "not at arm's length" means persons acting in concert without separate interests or who are related.
Related persons are individuals who are related to each other by blood, marriage or common law partnership, or adoption. Examples of blood relatives include grandparents, parents, brothers, sisters, and children. Examples of persons related by spousal relationship include the grandparents of a spouse, the parents of a spouse, the brothers and sisters of a spouse, the spouse of a child, and the spouse of a grandchild. Generally, in determining arm's length relationships, common law partners are treated in the same way as legally married spouses. Adopted children are treated in the same way as blood-related children.
Related persons also include individuals or groups and the corporations in which they have a controlling interest. Persons related to these individuals or groups are also considered related to those corporations.
For more information on arm's length, see Interpretation Bulletin IT-419, Meaning of Arm's Length.
(CRA : Charities Glossary)