It's as simple as if they had paid the rent. You enter the total rental income actually received in the tax year. If you actually received $0 in rental income, then you enter $0 in rental income and press on.
since all rental was to family members at below-market rent, every day rented to them is considered a personal day. thus the residence is treated as if it was your home. you report no rental income and no rental expenses. if it is a qualified residence, first or second home, claim mortgage interest and taxes on schedule A.