It's perfectly normal to feel a bit hesitant, and it would be just fine if you didn't want to as well. People will offer you various kinds of help - none of that obligates you to accept if you don't feel comfortable with it!
But it does sound like you want to have a break. I'm not sure of advice on supply as I didn't try expressing until a lot later - sure someone will be along with good advice in a moment. In the meantime, I found kellymom.com/ pretty handy for advice about breastfeeding.
Oh - just realised I do have one tip about expressing! Washing the pump/bottles etc can be a pain in the arse. If you are expressing in order to "take a break", then it's no break at all if you have to spend your very precious non-feeding/non-sleeping/non-changing minutes arsing around with sterilising stuff, expressing, then washing and sterilising stuff again, because this is all a lot more work than removing a bag from the freezer, defrosting it, and having a nice cuddle with baby while feeding him/her.
Ask your DH/SIL to take care of all the pump/bottles, so that ideally all you have to do is extend your hand for the nice sterilised and already-put-together pump, express whatever you can (don't be surprised or worried if it isn't much, a lot of women find that expressing doesn't get much out compared to what your baby can get by feeding direct), and then hand back the expressed milk for someone else to put away (fridge, freezer, whatever). Otherwise it will end up being more work for you, rather than less, and you won't enjoy your time out.