i dont really have much advice because i have left DD alone already quite a few times. It was important for me to have her get used to other people at a young age and get used to the idea of being away from her. But it really is just a matter of doing it just to see how he reacts, maybe your SIL can do something really fun with him and he won't get too bothered.

Don't push yourself if you aren't ready, because what is the point of being away if you are too worried you wont have fun.

But it really is nice to see their smiley face after not seeing them for a couple of hours and you get the biggest cuddles