Selena's Sister Suzette Blasts Brother A.B. After Cryptic Social Media Post

Selena Quintanilla's siblings are engaged in a war of words about an alleged legal action ... because while her brother says he's involved in a lawsuit concerning other members of the family, they're calling BS.

Here's the deal ... on Tuesday, A.B. Quintanilla III took to Instagram to tell fans there's a lawsuit involving his sister, Suzette Quintanilla, and "facts and circumstances" connected to their late father, Abraham Quintanilla.

He says the alleged lawsuit is part of a process that began months ago ... but the reasons for starting that process have been years in the making.

A.B. says, "There is years of history. There are facts. There is evidence. There are circumstances. And there is much that is still unknown."

He finished off his message to fans by adding that he will speak when the time is right ... with clarity -- and with evidence to support what he's saying.

Suzette waited a day to respond ... but his back in a joint statement with their mother -- Marcella Quintanilla -- Wednesday morning, in which she claims she hasn't been served with a lawsuit or received any legal documentation regarding this alleged legal action. We've also searched for the case and tried to reach out to A.B. about it ... nothing so far.

Suzette and Marcella -- the latter of whom says she's only breaking the silence to defend her husband, who is no longer around to defend himself -- add that all "insinuations" made by A.B. are completely false.

The statement denies any allegations of theft against Suzette ... and also denies that she intends to sell Selena's personal belongings. Marcella and Suzette say they are "unaware" of any wrongdoing by Abraham that backs up A.B.'s claims against him.

While they're clearly frustrated with A.B., the two ladies add, "We love A.B. He is our son and brother. But loving someone does not require us to accept false accusations." Once again, it's worth noting A.B. said nothing about theft or the sale of Selena's things in his social media message ... it's unclear whether Suzette and Marcella have inside knowledge of his grievance or if they're guessing.

If you don't know ... A.B. and Suzette Quintanilla are Selena's two older siblings -- and the only two other children of Abraham and Marcella.

A.B. broke the news of his dad's passing back in December ... posting a tribute to him on social media.

The family held a private service for Abraham back in January ... limiting attendance to only close friends and family. Even with those limitations, about 400 people attended the service -- which included performances by Spanish-language singers and a video tribute to Abraham's life.