This is difficult question to answer directly. It is most helpful here to look at what the reader does know from this chapter and the surrounding context. It is in chapter 24 that the heirs finally combine their clues to discover what they "don't have" which is "what counts." The clues are the lyrics to "America the Beautiful" with a few missing words. The missing words spell out Erica Berthe Crow. Under the pressure of time, several of the heirs repeat that the "answer" is Berthe Erica Crow. She finally exclaims in a moment of nervous pressure (which has been building since before the evening started):
The answer is Berthe Erica Crow...I am Berthe Erica Crow. I am the answer and I am the winner. (156)
It is difficult to say that Crow here is lying. She is not, in fact, the murderer. However, at this point in the game, it is unclear if the "answer" to Westing's game is even to discover the murderer. She may not be claiming to be the murderer, she may simply be claiming to be the "answer" to Westing's riddle.
At the beginning of the next chapter, she accepts arrest willingly, for murdering Sandy through his flask. It was not poison, as suspected, that she put in the flask, but rather it was lemon juice. So again, it is unclear (at this point) if she believes that she killed Sandy, or if she is simply speaking out of nerves and the fact that all the evidence points to her.
Later the true "answer" is discovered and the game is won by Turtle. Ironically, no one was correct. Westing was not dead. The "answer" did not include finding his murderer, but rather, discovering his four identities. In the end, Turtle never reveals to anyone that she won.
Crow, Westing's ex-wife, released of all charges, goes on to marry her love Otis, and works (at the soup kitchen) until she dies. Though her connection to Westing is made known, it is not made known what her intentions were on the night of the final clue reading. So it is difficult to say with certainty that she is "lying." Certainly, she is incorrect in her statements, but she might have believed that what she was saying was the truth.