The only way you could put this allegation to a conclusion is by exhuming the graves of the twins take DNA samples from their teeth then compare it to Colin's DNA.
I doubt anyone would be allowed to do this. But take on the challenge if you want to 
You are forgetting something. The DNA of at least one of the boys was found in the moderator. The main contributor in 2 tested areas was one or both of the boys. The people doing such testing didn't notice any similarity to Nevill's DNA. The MacKenzie report predated the DNA testing by many years. As part of the process of figuring out who the main contributors were they looked at the DNA of Neivll's relatives as well as Sheila's birth mother. There was no commonality seen between the DNA belonging to the boys and Nevill but rather simply commonality with Sheila indicating a relation to Sheila. That is how we know it was the boys. So there actually is evidence suggesting this wacky scenario is not true. But that is not surprising since the sole basis of the allegation was the simple fact that the boys had the same blood type as Nevill.
Sheila had type A blood and the boys type O blood so what blood type did their father have? Handy 8th grade science provides the answer A, B or O. Isn't that helpful that only eliminates the 2% of UK men who had AB blood as their father... So just based on this one factor 98% of UK men in existence when they were conceived could be their father. What blood type are you and were you over 16 at the time they were conceived?