Well theres always the "family discount" (Inflate the price 100% then give them a 50% discount) hehe. I own a repair business myself (auto-Truck) i ran into the same problem I dont charge labor but i do charge for parts.....and theres always the " Im very busy and i dont know when i can get there " excuse then they go elsewhere when they are tired of waiting for me.
Only ones that get away free are Mom and Dad. They raised me, fed me , clothed me and paid for my education, they did without so i could have. The least i can do is help them when they need it, espcially that they are retired on a fixed income.
Id guess in your case you have to make a decission, but remember, if they went somewhere else, they WOULD pay.