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Well, there has been a further development. A man who was the brother of the suspected aunt in this scenario (so would be my uncle) has done a DNA test too and has also come back as a close match - albeit not as close as his sister.

So I now have the following results:

BC (Suspected FC1R) and I share: 7.7% of our DNA, 543.2 cM, 25 segments with the largest being 71.1 cM.

PW (Suspected FC) and I share: 16% of our DNA, 1,130.7 cM, 37 segments with the largest being 88.1 cM.

PI (Suspected Aunt) and I share: 30.4% of our DNA, 2,153.5 cM, 41 segments and the largest is 166.3 cM

DP (Suspected Uncle) and I share: 22.6% of our DNA, 1,599.6 cM, 38 segments and the largest 136.9 cM.

Further to this, the man we suspect who may be my half-brother (by proxy of all the circumstantial evidence) is also doing a test and his result should be available this week.

What I am curious about is the DNA testing. I know DNA ranges can fluctuate anyway naturally, depending on the nature of the relationship and the sex of the person involved but both the potential aunt and uncle either come close to the 25% DNA / 1,750 cM threshold - or exceed it; however, their father died 10 years before I was born, so there's no possible way they and I could be the half-siblings. As such, if the assumed half-brother comes in around the same percentage, presumably there'd be no way from those numbers to distinguish the nature of all our relationships other than circumstantial evidence - so is there any kind of DNA test that he and I could take that would definitively tell us whether or not he and I are cousins or half-brothers?

Unfortunately his/our father is dead, so it's not an option for us to use him as a basis for comparison. My mother is dead, so I cannot get a sample of her DNA to use that to distinguish, and his own mother is completely in the dark about what is happening as they're concerned it will upset her - so she's out of the picture too. Is there any way to know for sure if there's no other way of obtaining DNA which would help establish specific differences?

What great progress & thanks for the update!
Knowing that your biological father is most likely the full sibling of your highest DNA match certainly points you in the right direction.

Well, we're almost 100% certain we know which of the three brothers it is through lots of circumstantial evidence, unfortunately he is dead so cannot be tested, and I am told it's unlikely that his other children would ever be tested - or get tested of their own volition either.

Although one of the brothers is apparently intrigued enough that he's considering testing himself. Which I guess means he's confident enough that it's not him. lol At least it would narrow it down to two.

Quote from: Nanna52 on Monday 16 May 22 23:17 BST (UK)

My half first cousins match me at 572 cM and 483 cM.  Whilst at the low end it is possible that the mother could be a half sibling.

Sorry to jump onboard but I've today found a new (unknown) match with...

1st – 2nd Cousin
8% shared DNA
578 cM across 21 segments

I've been in touch & will wait for a reply as to connections prior to jumping to conclusions 

I can't recall if AndyH81 actually listed the % shared DNA & segments with his matches?

Annie

BC (the daughter) and I share: 7.7% of our DNA, 543.2 cM, 25 segments with the largest being 71.1 cM.

PW (the mother) and I share: 16% of our DNA, 1,130.7 cM, 37 segments with the largest being 88.1 cM.

Also depends upon being tested with the same Company.

My Ancestry DNA matches have quite different cM figures on the comparison websites.

DNA Painter gives your 572cM as a 1/2 First Cousin at a 83% Probability and the 483cM the same at 89%

If you have not used their WATO tool, give it a go and see the vatios hypothesis’s that it presents

I think the percentages the two of us are referring to might be two different things? Yours seem to be saying the percentage of probability of X relationship. The percentages I am talking about is the total percentage of DNA that we share.