Sorry for Your Loss. Reprints were from 1888 thru 1907. On the front cover of reprinted issues is the small word "reprint " near the bottom of the front cover. From what you said I think the early issues you have are originals (most reprints are in mint -like new condition). Your money issues are 1913 back to your 1897 issue. Those years can bring good money on E-Bay. The rest of the collection has some value but would take a long time to sell because of the millions of copies that were printed each month. Early (pre 1915) issues with attached maps or supplements are also valuable.
I hope this info helps you out...
1960 issues sell on E-Bay for around $2.00 each while 1970's only 50 cents each. I would discard 1970 onward. Map values drop for 1960 on up to today. I would keep 1888 to 1957.(the late 1950's and the 1960's maps were basically a world atlas series). Keep the maps with their magazines when you sell the magazines.
For reprints (you'll be most concerned with the fifth type of reprint):
Hi Pat. So sorry for your loss. I hope your brother enjoyed his hours of collection with NG. It takes a certain kind of passion to embrace this.
If you are interested, I would be interested in purchasing everything prior to 1913. If you have settled on a price, and could take a few pictures of the magazines, I would love to talk with you.