So as you've seen from these responses, the answer is: Serena maybe, (in what sport is the greatest of all time NOT hotly debated? Maybe ice hockey), Venus definitely not. She had the misfortune of still being in her prime when her sister entered hers.
Interestingly however, Serena retired last year at age 40, while Venus is still out there at 43. She's obviously not the player she used to be and is now ranked #554 in the world. But she will still get "wild cards" (tournament/sponsor invitations) probably from almost any tournament she likes, and she did indeed take a wild-card slot at the WTA tour event in Birmingham, UK last week and didn't embarrass herself - she beat the world #67 in the first round and took the world #17 to 3 sets (hill-hill in a race to two, in poolese) in the second round.