I have written about the importance of testing siblings on any number of occasions, each time as background to a story. Here is another one from the last few days.
I received an email from a fellow named David, saying that he had seen indications on GEDmatch that we may be second cousins. He gave me a list of eight family surnames, none of which match any of mine.
I expected to tell him that the matching segments are likely from two or more distinct common ancestors, but first I looked at our kits on the GEDmatch One-to-One tool.
So I looked at David's top matches and one of my paternal second cousins stood out.
I also recognized one of David's surnames as someone on that cousin's other side - I think a maternal grandmother.
This second cousin has a full brother who has tested as well as a first cousin, who is also my second cousin. Here is the brother.
David's match with their first cousin is not worth mentioning.
So I wrote all this to David, bcc'ing the female cousin, so they can examine surnames. (This particular cousin is good about that.)
But my point, of course, is that had the sister not tested, all David would have had is the brother's second-cousin-once-removed level match. Would he have followed it up? Who knows!
Test first cousins. Especially test second cousins. But don't neglect the siblings. It matters.