You say they are not in the register but it's not possible to determine that with certainty. The most likely possibility is that the name is mis-spelt. If you know the date of birth, just search on that along with first name or maybe birthplace. There will be lots of possibles to search through but the correct one may well jump out at you with a mangled surname. The next likelihood is that their death wasn't recorded in the register, as you say you'd have to wait until their 100th birthday to find them then.
Thank you for your reply.
Both my father and his sister will be in the National Register
somewhereSearching on the DoB produces no results because they were both born less than 100 years ago so the records are officially sealed. Searching on given name but no surname produces no results for the same reason. If the surname has been mis-spelled, I've no way of knowing which of the 22 alternatives has been used or if a new one has been created.
My father
should be included because he died well before the cut off date. His sister died recently so she won't be there. She also married. I tried her under both her maiden and married names. Again, no results.
I suspect that if I find one, I'll find both.