Michael Jackson's alleged illegitimate daughter just suffered a devastating blow to her far-fetched claims ... because the woman who supposedly gave birth to her -- Diana Ross' sister -- says it's all a bunch of BS.
TMZ just spoke to Dr. Barbara Ross-Lee, who tells us she's pretty damn sure that Mocienne Petit Jackson "is not my daughter" -- adding MPJ is "delusional" and calls the situation nothing short of "bizarre."
Dr. Ross-Lee also points out a major hole in MPJ's story -- saying it would have been impossible for her to have conceived a baby with Michael back in 1975 ... because she never even met the guy.
Dr. Ross-Lee says there was a time when MPJ "was really harassing me at my office" -- and was forced to have her employer contact the police to get MPJ to stop.
According to Dr. Ross-Lee, she responded to one of MPJ's emails a few years ago to tell her she was mistaken and to wish her well.
As we first reported, MPJ filed documents yesterday requesting a judge allow her to take a DNA test to prove she's MJ's daughter ... and therefore lay claim to part of his fortune and be granted custody of his kids.