As if things weren’t already going badly for the prosecution in Michael Jackson‘s case.

Motions filed in the last 24 hours indicate that Jackson’s teen accuser’s sister, who was 16 at the time, had some kind of romantic affair with a 20-year-old Neverland worker.

And to make matters worse, this worker, named Angel Vivanco, is being summoned as a defense witness. He apparently will testify that the girl told him her mother and her mother’s then-boyfriend, now husband, meaning Janet Arvizo and Jay Jackson, were “planning something big.”

The girl also apparently called her mother “Psycho Mom.”

Here’s what makes this situation uniquely bad for the D.A. On the stand, in her own testimony, Janet Arvizo said she did not know Vivanco. Then, when phone records were discussed about 10 days ago, they indicated that calls to Vivanco were plentiful from Arvizo’s home after the family “escaped” from Neverland.

Arvizo had said she had no further contact with anyone from Neverland.

Compounding this would be the daughter’s testimony that she never left her room in the guest suites at Neverland while her brothers played with Michael Jackson. Vivanco’s testimony, if true, will place her with him most of the time. It may also address why Janet Arvizo really felt so much like her “family was out of control” while at the ranch.

But the Jackson defense will have a big problem if there is testimony that Vivanco’s romance with Arvizo’s daughter involved sex. She was underage at the time, putting him in jeopardy since he was over 18. The defense will not want to endanger its own witness’s legal status.

District Attorney Tom Sneddon is working hard to keep Vivanco’s testimony out completely. As with Vincent Amen, another proposed defense witness, Sneddon is working against a mountain of evidence that the Arvizo family faked their claims against Jackson and planned to extort money from him, one way or another.

Meanwhile, Jackson got a semi-good day out of former defense attorney Mark Geragos on Friday until his current lawyer, Tom Mesereau, threw a monkey wrench into the mix.

Mesereau assured Judge Rodney Melville that he had a “full waiver” of client privilege from Jackson, so Geragos could testify in this case. It was a good plan, and for a while it worked.

Fending off constant objections and interruptions from assistant District Attorney Ron Zonen under cross examination, Geragos took responsibility for much of what went on between Jackson and the Arvizo family. He admitted to ordering surveillance of the family by his private investigator, Brad Miller, and effectively portrayed Jackson as a sympathetic, benign lover of children.

Most importantly, Geragos was the first person on the stand in all these weeks to question Zonen’s characterization that Jackson “sleeps with children.” “You make it sound sexual,” Geragos said. “It’s not.” For once, the jury looked relieved.

But then came the problem: When asked about something that happened after Jackson’s arrest, Geragos said he could not answer. Mesereau informed the court that the full waiver was limited to everything that occurred before Nov. 20, 2003, when Jackson surrendered to police. Since a limited waiver was not what Mesereau had vouched for, Melville dismissed the jury. He accused Mesereau of “misrepresentation,” and Mesereau apologized lamely. It was a bad moment for Mesereau, who nevertheless seemed pleased that he’d gotten so much of Geragos’ positive, believable testimony.

The Geragos issue is now put off until next Friday, May 20, when both sides and Geragos will offer opinions on what to do when this sort of thing happens. As Melville said, he’d never had to deal with this before. “I trust you haven’t either,” he said to Geragos.

Melville continues to be a favorite with the press as he tries to keep what could be a kangaroo court on track. He deftly handled the overheated Zonen when the prosecutor’s overzealousness with Geragos got out of hand. “Go down two degrees,” Melville said. “I ride horses. When they’re too hot, we like to let the head relax. Relax a minute.”