Although a mortified Elena Dementieva did not mean the words as they sounded, Venus Williams never wants to hear any hint of the vile suggestion again. Dementieva said on Thursday that the outcome of tonight's third all-Williams final at Wimbledon would be "a family decision". Old wound. Still tender. Ouch.

Except that, speaking in her third language, the Russian insisted later that she was actually talking only about how difficult she imagined it would be to play against a family member, and what a unique "family situation" the Williams face: siblings playing off for the biggest title that once again one is guaranteed to win.

The problem was that the normally placid Venus only heard the clumsy quote second-hand, and took immediate offence to the American reporter who revisited the ancient but still sensitive subject of whether a family conspiracy is hatched before the sisters meet on the match court - as they will for the 16th time.

"The main thing is that I find the question pretty offensive because I'm extremely professional in everything that I do on and off the court," said Venus. "I contribute my best in my sport, and I also have a ton of respect for myself and my family. So any mention of that is extremely disrespectful for who I am, what I stand for, and my family. That's pretty much how I feel about the whole subject."

Indeed, it is some time since the topic has been raised, and five years since the siblings met in the most recent of their six major finals (of which Serena has won five). That was at the All England club, when Serena needed three sets to beat Venus in the final for the second consecutive year. Their most recent meeting in any tournament was in Bangalore, India, in March this year, where the younger sister broke a 7-7 career deadlock. In major finals, injuries, indifferent form and the draw itself have often intervened.

This time, Serena has joked that she would try to sabotage Venus at the breakfast table by scoffing all the Weeties. The doubles partners are, as usual, sharing rented accommodation near the Wimbledon village, regularly ordering Chinese take-away, sharing everything except too much talk about tennis.

The quieter and more thoughtful Venus prepares for her matches by reading a book, the extroverted actress Serena prefers to watch a video. Venus does not cook, Serena does, but more often at the vast home the pair share in Florida than she has during this Wimbledon fortnight.

The sisters have always been extraordinarily close, while somehow managing to separate the personal from the professional. Serena loves the freedom and attention that comes with being the youngest in a big family, while Venus has always been aware of the role she plays as the big sister, but these days does not expect quite so much from her high-spirited friend.

"I think that we share even more things together off the court. We definitely rely on each other's advice even more," Venus said, of the evolving relationship. "You know, I was a big sister. I was kind of like: 'You never make mistakes'. So then I think sometimes it put pressure on her to be perfect, too.

"Not that I'm perfect - but I'm just really like a nerd! She wasn't exactly that way, so I think we both now know our roles in the relationship and we support each other.

"I think she's definitely matured in a lot of ways. I don't really worry about her that much (now). I respect the decisions that she's gonna make. I don't worry ... And on the court, she's always Serena Williams. She's tenacious. She's got every shot no matter what. You can't bet against her."

The pair had a doubles semi-final scheduled for yesterday, and for all the times this situation has occurred in the past, it remains remarkable. How, Serena was asked, it is possible to pursue something so badly without thinking, at the same time, that she is denying it to her sister?

"Oh, it's easy, especially with sibling rivalry," she replied breezily. "It's like, you know, I personally want everything that Venus has. So (if) she wins a trophy, I would desperately want it. I would be really eager to have it. We're good at this now. We just leave everything out on the court. This is the finals of Wimbledon. Who doesn't want it?"

Undeniably, it is the appropriate climax between the game's two outstanding grasscourt players, despite seedings of seventh (Venus) and sixth (Serena). Whether the latter still feels she can overtake her sister's two-Wimbledon lead, she said was a moot point, insisting cheekily it was the grand slam total that mattered, (and not needing to mention that she leads that count 8-6).

Both have been serving brilliantly, if differently, for Venus has only one speed - express. Serena's is a slightly more varied game, but against her sister the reactions of the more demonstrative sister are usually more subdued. Perhaps for the same reason, few of their matches have been classics, and only five of the 15 have gone to three sets.

However this one ends, their father and coach will probably be back home in Florida, or at least en route, for Richard Williams says he has never watched his daughters play each other, and never will. "Emotionally, that's too much for me," said Williams."I can't watch it. I'd go nuts. I want both of them to win."

Instead, he said, he habitually books a flight whenever an all-Williams encounter is confirmed, as it was on Thursday when Venus beat fifth seed Elena Dementieva in straight sets, and Serena beat Chinese wildcard Zheng Jie in similar circumstances on the court where the duo have shared six of the past eight Wimbledon titles and are now guaranteed another.

As Serena explained it, there is no decision to be made. "This is a grand slam, This is history," she said. "We are both trying to make our mark."

As Venus remarked, once into the final, "it's every Williams for themself".