Beth Twitty also tells The Early Show co-anchor Hannah Storm she's heading to Holland, where a former detainee in the case, Dutch teen Joran van der Sloot, is attending college.
Reacting to an interview van der Sloot gave to the magazine show "A Current Affair," Twitty cast van der Sloot as coming out with one lie after another.
She also repeated her contention that her daughter was sexually assaulted by van der Sloot.
Holloway vanished at the end of a graduation trip to the Caribbean island after a night of drinking with friends.
Van der Sloot, 18, has acknowledged spending the evening with Holloway before her disappearance, but denies any wrongdoing.
Holloway was last seen leaving the bar with van der Sloot and two Surinamese brothers, Satish and Deepak Kalpoe, early on May 30.
In the interview with "A Current Affair," van der Sloot admits he lied about the last time he saw Holloway on the night she vanished, but insists she was fine when they parted ways.
"When her parents showed up at my door with her picture, I didn't even know who Natalee Holloway was," he said. "I didn't even know her name."
To that, Twitty told Storm, "There have been so many lies that have come out of Joran van der Sloot's mouth that it's so — sometimes I think the only facts we have are all of his lies. It's just incredible that he is going to make a statement like that, that he does — did not even know her name."
In his interview, van der Sloot said: "I kissed with her (Holloway). But neither me, Deepak or Satish ever had sex with her. And no one ever said otherwise."
Asked by Storm if she believes Holloway was assaulted, Twitty said, "Absolutely. And you know, that is strictly coming from Joran's statements. He has admitted this. He admitted this to the interrogators. He admitted this in front of 10 witnesses in the early morning hours of May 31, of the sexual assault that he committed against Natalee. So, yes, that is a fact.
"The family was there as witnesses to (the statements)."