If you aren't saying that they knew of her funeral and didn't attend (your post is not really clear) I would just put a note in to say that sadly you lost your wife or she passed away or what ever terminology you wish to use, in July of this year and you feel that you can no longer cope with such things as sending christmas cards; that you feel sure that they will understand. Just for interest Lord Mountbatten did the same after he lost his wife. Many years ago I did a very posh afternoon tour. We had time in Stratford and then a private tour of a local small staely home. The Mountbattens had been friends of the family (as had the Royal family been friends of the older generations of this family) and they showed us christmas cards including the last one from Lord Mountbatten.