It shouldn't be a problem, I took my 18 year grandaughter in as my guest and I only enquired if she could go in with me as she was a teenager but normally they won't check. As long as you have your Diamond Card which they supply to enter the door then all three of you can enter with no problem. The Diamond Room is on Deck 14 off the Viking Lounge.
It all depends where the wife's sister is sleeping!. If she is in the same room as you then, yes, she will be allowed in as your guest. If she is in a different room then, no, she will not be allowed in. There are two concierge lounges on Independence - one is on deck 10 in the middle - a soulless place with no windows, and then there is the Diamond lounge up by Viking Crown lounge which is very nice indeed with a small outside seating area.
I don't understand that how by not sleeping in their cabin would make a difference, do they check you off as you go in? They certainly didn't when I used the Lounge. If one person has an entry key then there is no problem I would have thought, she can join them especially as she is one of their party and single. It would be different if they wanted to take a big crowd of people in there with them. Anyway, I'm sure if they ask then she will be allowed in, the Concierge was very accommodating and the service between 5 pm and 8.30 pm was excellent.
I am just quoting the Rules.....I would not like to set their expectations that she will get in and then she does not.
My two pennuth! It also depends how many 'Diamonds' are on board. I have witnessed people challenged when the lounge is overcrowded. On my last 2 (short)cruises their were few in the lounge on any night & was never questioned, on others we have been 'computer checked' on first night arrival.