I have a half sister with whom I share 22% DNA ( I think it's not 25% because I don't think they compare all of the DNA) . Hope this helps.

I’m trying to see how the numbers work out with different scenarios.

... brothers have the same biological father and mother, they come from the exact same DNA pool. There is 0 difference there... a full sibling (identical, fraternal, or regular) is a great sperm or egg doner, if you're looking for DNA.

Could a child confirm which one is the dad with a dna test? If they didn’t tell her about the donation?

... each brother gets 50% DNA from his mother and 50% from his father. The same 100% DNA pool. However, each brother would have pulled a slightly different 50%... making them unique. You could have a father hold 1% Italian heritage, and pull none of it - but a sibling pulls the full 1%. It's random. The child and brothers providing DNA samples would ID the father, for sure. The DNA tests can get very detailed, especially with the subjects all alive and providing samples.

The child of one brother would share about half that, or 1250cM. My wife's sisters share differing amounts with her. One is just under 2300, the other is over 2400. Both of them have more of their mother's side than my wife, who has a majority of their father's DNA.

Thank you for giving me a new lead.

Picture a deck of cards, divided into Red (father) and Blue (mother). Shuffle the cards, then deal out 23. That's you. Put the cards back, shuffle, and deal another 23. That's your sibling. Same deck, some cards are the same in both siblings, but not all.

Hope that helps. Feel free to p.m. me if you have more questions. I'm not an expert but have worked with DNA quite a lot doing genealogy.

Very visual.

... with both our parents and all of our non-identical twin siblings.

Cut that about in half for each subsequent generation -- 25% shared with grandparents and uncles / aunts, etc.

If you're asking for the amount of DNA shared with an uncle (a sperm-donating brother of the father), it will be 50% with him because he's the biological father. The "father" will instead be the biological uncle, sharing about 25% DNA with the child.

My DUers are hitting this one out of the ballpark!

The couples each had a son. One couple had Ray C and the other couple had Jay C.

Wonder how close in DNA the cousins Ray and Jay are?

This happened in my husband’s family.
He has always called the cousin ..a double cousin.

Tikki

Interesting family dynamics,