Okay, well I get where you're coming from, but since you know about all this now, I'd talk to your in laws about it now. Just call them up and say hey, I just realized that my cousin's baby shower is on Sat and I'm sure that is when dinner is planned for too....blah blah blah.
Just put it out there and work around it. Tell them that you are comfortable leaving the shower a little early but still won't arrive until 5pm. Hopefully they will wait for you and if they don't, they will at least know you're on your way.
And yes, now that you have a child, holidays and in-law relations become WAY more complicated I'm afraid. :-/ But, I think you have a better chance of making everyone happy if you just lay it out there now so they have time to talk it over and plan it out.