Dementieva, who was defeated in straight sets, blamed language difficulties for her initial comments being taken the wrong way.
In the post-match press conference she said: "I cannot imagine myself playing against someone from my family.
"It's really hard and for sure it's going to be a family decision."
BBC tennis correspondent Jonathan Overend said: "Dementieva has a little bit of history on this subject.
"She lost to Venus in the quarter-finals in California in 2001 and said afterwards: 'I'm not sure what father Richard Williams thinks about it. I think he will decide who will win tomorrow'.
"Maybe she does feel that there is some outside influence. But you only have to look at the head-to-head between the Williams sisters to see there is not great evidence where Venus wins one match then Serena wins the next."
Serena beat Zheng Jie to book her final spot and has beaten older sister Venus both times when they met in the 2002 and 2003 finals.
She said: "It's easy with sibling rivalry - I want everything Venus has, and plus we're good at this now - we just leave everything out on the court. Who doesn't want to win - this is Wimbledon."
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