Siblings Passing On Same Date, Different Years

There is no numerological significance within traditional Pythagorean numerology. Numerology has no calculations related to death other than things that would also relate to other things — such as dates.

Every date can be calculated for meaning. Generally, the date would relate to something, like a birth date or marriage date. But if it's a date of passing, then numerology has no calculations specifically for that.

The above is not to say there is no significance. Of course there is. It is significant to you or you would not have asked the question. But what the significance might be is outside the realm of numerology.