This will sound harsh, but neither your MIL nor your SIL is trustworthy at this point. SIL wouldn't be unacceptable if she were a responsible, caring person (she would have built up a good relationship with you and her nephew already), and MIL is totally out of bounds trying to give authority over your child to someone else without your prior permission! And not to sound totally cynical or paranoid, but there has to be more to this...maybe MIL just doesn't really want to come and care for your son all by herself, or maybe she is subconsciously trying to set up a disaster. I know that's going out on a limb, but honestly, I can't imagine a loving grandmother even considering letting a sweet 4 year old be in the hands of someone who is prone to such temper tantrums (I mean, if SIL's temper scares MIL, imagine how quickly SIL could become enraged by some innocent act from a 4 year old!). Your child won't be safe with either one. You can't trust your MIL now, not now that you know she will go behind your back and give your child to an irresponsible person. I'm so sorry you have to deal with this! Take it as a gift from God that you found all this out ahead of time, and take your son with you. You can figure out another trip later. Oh, I hate saying that! Do you have any friends who could keep him? Best of luck to you. One last thought - I wouldn't sway even if MIL gives in and says she'll keep him. She just can't be trusted, IMO. And she needs to learn the boundaries of acceptable behavior!