Things appear to have gotten shakier for R. Kelly’s defense as a bombshell witness is set to take the stand against him today. Last week’s testimony from a prosecution expert has already helped show jurors a mole that the singer’s lawyers said was absent from the body of a man resembling Kelly in a sex video. Defense attorneys have shown that Kelly has a mole on his lower back and suggested that the man shown having sex with an apparently underage girl doesn’t have the mole. But after slowing down and freezing the footage, forensics video analyst Grant Fredericks told jurors: “There’s a mark on this man’s back (in Kelly’s police photo) about the exact same size and position as the man in the video.” Lawyers for Kelly only got Grant to concede that he can’t say for certain R. Kelly is in the footage. Prosecutors hope that re-scheduled witness Lisa Van Allen will remove all doubt today. Van Allen, 27, was first expected to take the stand last week to say she and the alleged victim, believed to be in her early teens at the time, had three-way sex with Kelly. Van Allen reportedly had another video featuring Kelly, but allegedly sold it to the singer’s handlers. The Georgia mom’s testimony was postponed after defense lawyers said a witness contacted them, who could refute her testimony and that they needed time to interview him. Van Allen’s fiancé Yul Brown says she’s prepared to testify anyway. “You have a child who was impressed with an R&B singer,” Brown tells the Chicago Tribune newspaper. He says Van Allen got involved with Kelly when she was just 17 and appears in the video for Kelly’s 2001 song “I Wish,” as Kelly’s hair braider. Van Allen’s fiancé says she “had a serious relationship” with Kelly, despite the singer’s marriage. Brown also confirms there was a tape in Van Allen’s possession: “Yes, a tape existed. I won’t say if I have any knowledge of what happened to the tape.” Both Kelly and the alleged victim of the child porn deny that they appear in the 27-minute recording. The singer faces up to 15 years in prison if convicted.