When people asked them questions, Annette and William described themselves as husband and wife. This was not an official status, and would never be one, legally. The two of them were blood related. Fraternal twins who had been removed from their parents and raised, separately, in the dubious care of the Umbrella Training program.
They had been in their mid teens when they reunited, and there had been no question about it. Blood, as they say, calls to blood.
It was easy to keep the nature of their relationship ambiguous while they were being educated. There were plenty of blonds. Plenty of same last names. People didn't ask, and they didn't tell. Besides, people assumed William was in some kind of relationship with Albert Wesker (he was, additionally), and people didn't think about Annette at all.
Annette didn't mind being overlooked. It gave her more time to work, and more privacy.
It was once they were adults, and given the independence of a home, and a life nominally outside of Umbrella that they made the decision to tell people they were married. After all, if they were living together, people would draw their own conclusions anyway.
It was as simple as a gold ring, one for each of their hands, and it was as good as done. Annette would have left it at that. William had insisted there should be cake. Annette had to agree that it was a nice cake.
Albert Wesker presented them with a pair of swords as a wedding gift. They named him Sherry's godfather.