Confirming a connection

Question

Ancestry connected me with 2 people that are siblings, (Unknown to me) .Our DNA match between me and the other 2 is 850 and 971 CM respectively. They are 10 and 20 years older than me as well.

Based on timelines and Geographical proximity to my Paternal grandparents, I'm left to wonder if my Grandfather was biologically my Dads Dad or not. My THEORY is that these 2 siblings Dad may be my biological Grandfather before he even met his subsequent wife and had children. These 2 would then fit right in on the DNA match percentage to me , the timeline makes sense, and their Dad was living in the small neighborhood of my grandmother during the time of my Dads birth.

My Grandparents only had 1 child (My Dad) 5 years into their 10 year marriage. (That alone seems odd)

I am able to trace DNA and known relationships to my Dads Moms family and my Moms, Moms family. So I do know who my parents are.

I have asked an elderly first cousin of my Dads to take a DNA test and he has agreed. Since my Dad and his cous share a grandfather, there should be some DNA connection to me and my Dads cousin. (I would think). If there ends up being no connection, how can I establish that these 2 siblings actually are the half siblings to my deceased father?. Right now besides the connection to these 2 children and that their dad was living close by to my Grandparents, I have nothing else to prove that their Dad is my grandfather (biological). Is there some other test , or something in our DNA results that would be more conclusive?

As a followup... I am a Male. The 2 siblings are 10 years apart. one Male, one female. The male to me and him match 971CM, I match 850CM to his sister

Answer 1

There is an easy test of whether your paternal grandfather could be their father. You and your male relative can get Y-DNA tests from Family Tree DNA (on sale through the end of August). If they don't match closely, your paternal grandfather could not be their father (because only men have Y-chromosomes, which are passed down unchanged, except for minor mutations, from father to son). The least expensive Y-DNA test should work.

If the Y-DNA does match, it does not prove that your paternal grandfather was their father. Another close male relative could have been involved. A mismatch, however, disproves the hypothesis.

Answer 2

I presume your Dad's cousin is on your grandfather's side. (If he's on your father's mother's side, then he won't be of use.)

With your father's cousin's test, you might have enough information to figure it out. The tool to use is the What Are The Odds tool by Jonny Perl at his DNA Painter site. You draw out your family Tree and put one of the siblings in all the places they might fit. Then you add the cM you and all the others who have tested share with the sibling The tool will tell you which relationships are possible, and if more than one is, then it will tell you how likely each is.

If your father's cousin's test is not enough to figure it out, then one other relative test on your father's side or even better, a relative of the two siblings' father should nail it.