Baron Davis’ sister is suing him for fraud and intentional infliction of emotional distress … after she claims he sold the house he helped purchase for her with no warning.

According to the lawsuit, obtained by TMZ, the NBA star helped his sister Lissa Davis buy a house nearly 20 years ago … but sold it out from under her this year, without a heads up.

Lissa says in the suit that she had a “life-threatening” health issue after the birth of her son back in 2008 … and asked Baron for help with money.

She claims Baron convinced her to sell her Northridge, CA home because he was going to buy her a new one through his realtors. Lissa alleges Baron asked to keep the title in his name for “tax purposes.”

Lissa says she then found a different home in Northridge that she sunk all of her savings into fixing up with her husband.

Fast forward to this January … she claims she found out from a third-party realtor that Baron was selling the house … and she claims he never said a word to her about it.

Lissa’s suing him for fraud, intentional infliction of emotional distress, and more. She’s seeking an unspecified amount of damages.