Sorry you are having difficulties.
The best thing is to take control of the situation.
Even though you are using Instant Book, You must message your guest and confirm that you have no kitchen sink.
You must also ask them to acknowledge the missing sink.
Something like:
"So that you may understand and have a good time here in the Bahamas, you do realize there is NO sink in the kitchenette.
Is that OK?"
At this point guest may cancel, but that is better than a disgruntled guest. You won't have to worry about this because everything is out in the open and you will find that guest who is not bothered by your offering.