A baby shower is supposed to be for the mother to be, or the parents to be. Yes, most items are for the baby, but since baby presumably still lacks the motor skills to open a box of diapers and warm his own bottles, Mom and Dad are usually responsible for these tasks and therefore can use the equipment.
I have heard of "Grandparent showers," but I don't care for them, unless Grandma is planning to adopt. Technically, a family only shower given by a relative is perfectly polite. Determining who is related at this late date is rather a moot point, isn't it?
At any rate, starting an inter-family war will not help anyone, as Mom21son, states.