Thank you everyone, for your support..
I have dealt with the situation. I called up a retired Police officer who knows the family well and me from my juvenile days. We decided to do it this way:
My cousine will tell my sick cousin that she will invite the horsestable-owner to let his grand-children go horsebackriding.
Then he will not return the horses. My cousin will forget about them and the Police in the area will not do anything if he calls them (This is highly unlikely, as he is not even aware of them right now).
It seems good!
But taking them by night would have been more fun...:p