People who have been able to compare their DNA to other close family members on a site like Ancestry know just how accurate that is. Parents share about 50% of their DNA with their children and, likewise, full siblings share roughly 50% as well. That comes out to about 3, 500 centimorgans, give or take. Almost needless to say, the more distant the relationship, the fewer cMs. This is a good checking source, if in doubt.
https://dnapainter.com/tools/sharedcmv4/1089
Worrying about whether an identical twin brother of someone might have been your father is rather pointless, IMO, as it is so unlikely to have been the case, It is only identical twins that have the same DNA. Fraternal twins are like any other full siblings when it comes to DNA. 50% of it will be different.
It may be pointless to you but I can assure you that when a mother just before she dies of cancer tells one of her daughter’s her father is really her grandfather that daughter does not think it pointless. In the above case the mother and her three elder daughters were living in the grandfather’s home while her husband was serving a prison sentence. There is very little chance the mother did not tell the truth, unless the daughter was born very premature or late but there was no hint of that.
Every DNA expert I have spoken to agrees it would be very difficult to discover the birth father via DNA.
The gray area involves ethnicity. While one company will identify you as 30% British, say, another will assign you perhaps 38%. All you can deduce from that is that you have a significant amount of British ancestry. Another company--not Ancestry--at one time assigned me some small bits of interesting ethnicity--and ended up taking that away for a reason I have yet to understand. But, if one can acknowledge a lack of complete accuracy in that aspect of ones DNA results, one can live with it. It is not to be confused with the accuracy of relationships between people, that's all. The reliable testing companies look at hundreds of thousands of markers, not just a few. You can bank on the results.
I think my views on ethnicity results are well known.
With regards to the accuracy of DNA
The population of London in 2018 was 8,787,892 the largest worldwide DNA database is estimated at 12,777,778 in other words half as much again as 1 city or if you compare this to the world population of 7.6 billion (in 2018) in other words a small proportion.
A lot of claims are made about DNA but at present it is still very much a tool in the genealogists toolbox that relies heavily on other tools to produce results.
Cheers
Guy
PS All the daughters (5) have had their DNA tested, the older generations are all deceased, mother, father & grandparents.