How do I get my siblings to realize that it is important to be with our parents during the holidays?

You didn't give many details, for example, do your parents routinely ask your siblings to come for the holidays? Do your siblings live far away? Are they not coming because of bad feelings, or for some other reason?

Of course, you can't force your siblings to visit, whether you think they ought to or not. What you can do is to simply tell them -- without being accusing or judgmental -- that it would mean a lot to your parents for them to visit during the holidays. It's possible that they are so wrapped up in their own lives that this has not occured to them. You could get the ball rolling by hosting a holiday dinner at your house next year, and inviting your siblings and your parents. Give everyone plenty of advance notice, keep it low-key and low-pressure, and let your siblings know how much your parents are looking forward to the event. With luck, and assuming the celebration goes smoothly, everyone will want to do it again next year, and you'll have established a new family tradition.