I would say to go ahead and give it a try, if it doesnt' work out, you can always change later. You know that he will be well taken care of by his aunt, because she is family and loves him. He is not just another dollar sign to her. I would say that you ask ehr to please not go to your mother in law letting her knwo all your business. find a nice way to ask this, of course. If it becomes a problem with your mother in law giving unwelcomed opinions, and trying to interfere, then speak with the mother in law and tell her that while you all lvoe her and respect ehr opinion, you will nto tolerate her contradicting you and your husband, or trying to take control of situations that have nothing to do with her. If it continues to be an issue, decide then what you would feel most comfortable doing with your son. If the aunt is doing a good enough job for you, and is the most affordable option, then i would say screw what your mother in law says. let her say ehr piece, and let it roll off yoru back.
You may want to consider, though, that the downside to family watching the child is they do love him, and they will let certain rules slide, allow him to get away with things you don't necessarily agree with, and may brush off some of your wishes, because tehy feel they are harmless, and want to insure his love and affection.
Good luck.
liz