Late Drop off WARNING VENT!!

[mimi]

I work with the parents when they have an appointment involving their child, but I do stipulate what time they can return their child as I will not wait for them.
Your SIL is out of line here. You have your schedule/routine and she doesn't respect it. The "we'll see" comment after you have explained your schedule needs unminds you and is disrespectful.
You have told her the time frame they can come. If they show up outside these times I would not answer the door. Obviously they don't respect your words so maybe they will respect this action.