Oh Harrysmum I feel for you. SIL has suggested everyone go away together for Christmas. Fortunately hubby is not keen so we won't be going. I've never seen so much of my in-laws since our son was born. One thing that works for us is that hubby will now visit his parents with our son alone giving me the chance for a lie in, or a few hours to myself at a weekend.
I think there just comes a point where you've got to be honest and say 'look, no offence, but we'd actually prefer to go away by ourselves' giving a list of reasons such as because of work don't see each other very much, like to spend some quality time just the three of us, don't relish the idea of travelling with a small child, I could go on. If you do feel pressured to go on holidays with other people/families then insist that you have your own apartments.
Other SIL wants us to visit her for a few days and hubby is keen on that one, but I will be honest with him and tell him I really don't want to go. That way at least he knows how I feel and maybe we can reach a compromise.
Just had a thought - what about investing in a video camera and sending the family regular tapes of your son? This might satisfy their need to see what he is up to, how fast he is growing and cut back on some of the trips. You could get them to do the same - send tapes with messages to your son and then they could feel they are interacting with him over the miles!
Families eh - aaaaahhhhhh!