Yes as long as:
1. The relative is a worker at that workplace;
2. He/She is chosen by the workers; and
3. He/She doesn’t exercise management functions.

Here’s the relevant part (Section 8(1)) of the OHS Act:

Mandatory selection of health and safety representative
8 (1) At a project or other workplace where no committee is required under section 9 and where the number of workers regularly exceeds five, the constructor or employer shall cause the workers to select at least one health and safety representative from among the workers at the workplace who do not exercise managerial functions. R.S.O. 1990, c. O.1, s. 8 (1).

Hope that helps. Glenn