Aunt nephew shared dna

[Yonnie3]

Grandpa died in 1978, the family never knew his family.  There is no paper trail or records that lead anywhere.  Except for this one cousin to the aunt that the nephew has no relation to.  It sort of begs the question as to who's your daddy?  Is it grandpa with no family?  Or is it the secret grandpa, grandma never mentioned?

Am I completely wrong with this conclusion or am I suffering from self-inflicted contusions?

Of course sisters aren't going to be an exact match, I understand this, otherwise DNA would be a useless tool if it showed all siblings as exact matches.  But DNA testing should show all siblings as being related, otherwise again, DNA testing would be a useless tool.  Are there any proven examples where DNA testing failed to show proven siblings as not related?  Or no relations being related?  And if so, how do we know a little sperm didn't sneek in there and certain person(s) didn't want to admit it?

[Gadget]

siblings would inherit 50 % from mother and 50% from father but this would not necessarily be the same 50%, etc, etc. See Mendel's work, for example.

[Gadget]

How much dna do you share with your aunt?

[Gadget]

You mention your siblings.  Have they been tested?

[Gadget]

I've just looked at the shared DNA between two of my cousins

I share 199.7 cMs with a 2nd cousin  and 226 cMs with a 1st cousin once removed.  They are from the same line.

When I compare them in My Heritage, they only share 19.3 cMs.


Gadget

[Gadget]

Another example - the 2nd cousin in the earlier example  shares 9.5 cMs with a known 4th cousin and I share 110.

Have you used/looked at this:

https://dnapainter.com/tools/sharedcmv4

[Yonnie3]

I share 2598.4 cM with my sister and 950.8 cM with my aunt.  Sorry the spam filter caught your response.

[Gadget]

What do you share with the 3rd cousin and  what does your sister share with 3rd cousin and your aunt?

Have you done a 'shared match' with the 3rd cousin and yourself/sibling. I've found that doing a kind of snowballing process helps a lot. 

In the example that I gave before.....

Another example - the 2nd cousin in the earlier example  shares 9.5 cMs with a known 4th cousin and I share 110.

......the 1st cousin, once removed shares no cMs with the 4th cousin, or at least not enough to show up.

Gadget

[Gadget]

Have you uploaded your results (and those of aunt/siblings) to other sites - gedmatch/my heritage/ftdna, etc.?  These site have more tools for comparing chromosome matches, etc. It doesn't cost anything and can be useful.

I did an Ancestry test but have uploaded to the other sites mentioned.