I have a feeling it will be Venus first. But i think both of them got a few years left in them yet. Fingers crossed.
Thats the worrying thing noone is really coming close to those two at the moment.
Don't get me wrong, I respect Venus as a player, but I do think she unfairly gets a lot of credit off of Serena's success. For example, many people will say, 'The Williams sisters dominate women's tennis', well, I think that statement would be true, if they talked about Serena as one person, but as for Venus, she hasn't dominated Women's tennis in a long time, and her achievements (barring doubles) is similar to that of Justine Henin.
On topic though, it will be a great loss to the WTA when those two, and the Belgians retire, in that case we have to pray for the revival of the middle 'forgotten' generation (Sharapova, Ivanovic, Jankovic, Safina), and hope that the young generation can be competitive (Wozniacki, Azarenka). We all know that in about two years time, the WTA is up for grabs for anyone who wants it, and I think it will be hard to predict who will dominate if anyone. Perhaps, as time progresses the distribution of slams will even out, hell, the WTA is far ahead of the ATP in this regard even now.