Hi,
This is pretty foolish and downright vulgar really.
The timelines of all this need to be explored.
George V was born in June 1865; therefore conception would be August/September 1864.
Nixa and Dagmar were engaged in the summer of 1864 in Denmark; and even if Alexandra was visiting her parents then, she would have had her newborn baby boy (Eddie) with her and probably the Prince of Wales too...
Also, there would have been Mama (Louise) & Papa (Christian) hovering around all of them, not to mention several servants.
Add to that was that Nixa was reputedly an honourable man and wouldn't have been dallying with his fiancee's sister....
This is just my opinion, of course; but I dare say someone here can do more of an investigation as to who was actually in Denmark in the summer of 1864...
Nixa and dagmar for sure, but were Alex and Bertie there then??
Larry