First of all, what type of computer do your relatives need? Are they power users? Or are they just going to be needing iLife, Safari, Mail, etc.? If there is nothing heavy-duty that they are going to be needing the machine for, I wouldn't even worry if iMacs are upgraded anytime soon - the G4 iMac will be more than adequate for their needs. Even when the iMacs do receive a G5, other upgrades, etc., that might be overkill for your relatives. Nice to have, but not absolutely necessary.
Regardless though, iMacs just recently received a pseudo-update, with the release of the 20", and the prices did drop by $50 or something like that as well. As a result, I don't see anymore price drops or updates anytime soon for the iMac, as I think the next update for the iMac will come in the form of a G5 iMac, which will be completely redesigned, and more expensive too (just my hunch though), which is another thing to consider - you could wait for the new model, but it may be more $$$ than the exisitng one, and more $$$ than you want to spend! We won't be seeing G5 iMacs until late spring/early summer, in my opinion, so I would say keep your order, as long as the G4 iMac will do everything your relatives need it to do. Just because a newer, faster model of a system comes out doesn't mean the one you have is crap and that your system magically becomes slower - know what I mean?
I doubt anything iMac-related will be announced at MWSF, so I wouldn't worry too much. Even if there is a minor update, (which is highly doubtful since there are SO many things due for updates before the iMacs! LCDs, xServes, PM speed bumps, iApps, iPods...), it would only be in the form of a speed bump, and really, is there a huge difference between a 1.25 GHz G4 and a 1.33 GHz G4? Not enough to warrant a re-order in my opinion.