Nicole Brown Simpson‘s sister is having a change of heart about the infamous murders … telling TMZ she now believes O.J. Simpson may have had help getting rid of the evidence.

Tanya Brown tells TMZ … she’s always believed O.J. killed Nicole and Ron Goldman on his own … but a bombshell new report has her reconsidering whether someone helped cover his tracks and discard bloody clothes and the murder weapon.

She tells us … “Someone had to have discarded that bag … and I don’t think it was O.J.”

As TMZ previously reported, a New York Times investigation raised questions about O.J.’s associate Joseph “Joey” Ippolito, whose brother claims Joey returned home the night of the 1994 murders with bloody hands and a heavy bag.

The brother, Sam Ippolito, claims he later dumped the bag in a dumpster … though the allegations have not been proven.

Tanya says she could see attorneys involved in the case brushing off a tip like this, accusing them of being too cocky and confident about winning.

Still, she’s questioning why Sam is coming forward now, especially since Joey is no longer alive to defend himself.

“Everyone wants their 15 min of fame,” she tells us, adding she’s not interested in speaking with Sam.

Tanya also reveals she’s finally had time to grieve her sister this year and is now sober … something she says has helped her become more open to the possibility someone else disposed of the evidence.

Meanwhile, O.J.’s former attorney Malcolm LaVergne has called for an end to the speculation, claiming the endless theories do little to bring closure to the families.

Harveyalso weighed in … saying the theory that Joey helped dispose of evidence may well be true … but stressing that any involvement would have come AFTER the murders, not during them.

O.J. was acquitted of the murders in 1995 but later found liable for the deaths in civil court.