Can references to apartments be relatives?

Yes, though the management company may also want to see a former landlord, if any, and/or a work reference to verify employment. And, depending upon the company, many don't care so long as you're not on a tenant blacklist, have had housing court actions, and meet the financial criteria for the apartment. Smaller landlords who self-manage and advertise on their own can be more flexible with requirements than larger management companies that may owe fiduciary duty to shareholders, hence more strict application of the rules.

Usually references are supposed to divide into two groups. Business references are non-relatives who can vouch for you, such as business acquaintances (your boss or supervisor), high school teachers or college professors, and in the second group friends or others who are respectable and employed as personal references. You should never use family as a reference.