I think you need to take the firm approach w. your wife and tell her you can't support what she's thinking of doing.
If she has the trust of her sister, to keep the little one for 2 weeks, that will be History. You can bet the sister
will feel betrayed, angry, hurt, etc. It is not right to step in the middle of it like she wants to do. It will create
bad feelings that will only escalate. Good luck and let us know how it goes.
AuntieJ