We are not the only ones ?

[Rwandrall]

Well I don't want to start shouting about the need for firmer evidence cos of the risk of offending your brother and his wife..

You know the conception occured, and you can infer the physical technicalities that must have occurred, but the actual technicalities were only known by those present, surely? Or did you record the act?

and even if it was recorded, he couldnt know, for example, what both...parties...thought at the time, or the reason for this conception.

Absolute evidence is impossible.