Oh dear! What a terrible position for them to put you in. I think the best way to handle this is to call your mother-in-law. If she told your husband to tell you, then she probably doesn't want to cause confrontation and upset you. I would kindly explain that you don't feel comfortable leaving him with a stranger for that long. Also let her know how upset you would be worrying about him if he was at home and don't that to be a distraction from spending quality time with her. Reassure her that they will get to spend plenty of time with you and Teddy will be out of the way. I'm sure she probably just doesn't understand how much it means to you. If you aren't owned by one of these wonderful furbabies, then you just don't get it.
Try to make her understand and I'm sure she will change her mind. Good luck and keep us posted!