I can understand why you'd want to avoid being ill and in labour if you at all can, but it's a bit mean not to see them over Christmas.
Keep the vists short, insist on hand-washing, anti-bac's, clean tissues for every sneeze etc. Drown yourself in Orange juice to boost your immune system and have the Vick's 'first defence' stuff on hand for after every visit.
TBH, though, even if you're exposed now, you likely won't be symptomatic until after you have child, and by that time, your milk will contain antibodies.
Are you planning not to let the IL's see the new born at all? They'll expose the baby then anyway - might be better for you to exposed now, so that you CAN pass the antibodies on.
I, too, have rellies that never warn me when they're ill. And it is damned irritating, especially when there's a good reason why I could live without being ill... (thinking of time FIL exposed me and DH to MIL's virulent both-ends-at-once, upchucking every half hour stomach bug two days before my wedding... and expected us to clean up after her, wash sheets from two days before etc)