This to me is a Christmas classic, inspired by seeing the second one I chose to re-watch the original and have to say that it is still the best. Yes you have to be honest and say that the chances of any family forgetting a family member is highly unlikely and that surely they would notice pretty early on. But they do a good job of setting the Kevin character up well within his rather large family.
This carries the same story as the sequel but is the original, it has all the Christmas values about being alone and the importance of family. It has some awesome comedic moments with the bumbling criminals in Harry and Marv played by Joe Pesci and Daniel Stern to great effect.
I have to say this is 5 stars for me, yes there are a few realism flaws within the plot but they get where they need to be and the comedy can still make me laugh even after all of these years, make it one of your Christmas movies this year if you have not seen it.