Well, seeing as I would never, ever, ever go on vacation with my in-laws... not willingly that is... I say "you snooze, you lose"... and you should take your vacation as originally planned. If Keith and I changed our plans every single time a family member or friend changed their mind... we'd never leave the house. It's your life and your precious vacation time... do what is right for you and not for someone else. If you go as planned, perhaps next time they'll take the commitment a little bit more seriously knowing that you will go without them. After a few not so pleasant vacation planning episodes... we now opt to simply tell family and friends... "this is where we're going and when we'll be there in the event you would like to join us"... and leave it at that. Sometimes people join us... however, more often than not they don't because they just can't make a darn commitment (or just don't want to go on vacation with us in the first place but didn't want to hurt our feelings... so we've given them the perfect out).
I never found there to be any dramatic difference in the weather conditions from January to February. I would be happy going either month... however, given the choice of the two...my vote would be for January... primarily because I'm not big on crowds and January tends to be less busy than February.