has sued her mother Constance for spending money on herself – money that the actress had been sending for the care of her brother Alexander, who has medical issues.
Constance Meester used the $7,500 a month for cosmetic procedures, among other personal expenses, the suit alleges.
"This whole thing's been really tough on Leighton," a source tells PEOPLE. "Her only concern has always been taking care of her brother. Leighton's really close with her brother and her dad – her parents are divorced. Leighton's a sweet girl – she cares about work, friends and family."
The Gossip Girl
actress, 25, claims her mother threatened to sue her for $3 million if she didn't raise the monthly allowance to $10,000.
On Monday, Constance fought back by filing a counter-suit claiming that she had sacrificed almost everything to advance her daughter's acting career, only to have Leighton break financial promises and even attack her last December.
"Leighton intentionally assaulted Constance by throwing her to the ground and repeatedly hitting her with a bottle," the legal papers say.
Constance also claims that Leighton reneged on an agreement to support Constance because the actress couldn't convince her brother Alexander to leave their mother and move to New York after he had recently undergone brain surgery.
"Since Alexander receives 24 hour care from Constance, no alternative source of income is possible," says the mother's counter-suit. "As a direct result of Leighton's breach, there are no funds to feed and care for Alexander."
Constance Meester has had legal troubles before. In the 1980s, she was convicted of helping a drug ring smuggle marijuana from Jamaica to the U.S. – and gave birth to Leighton while in a Texas halfway house.
Another woman also won a restraining order against Constance earlier this year after alleged harassment.
The most recent lawsuit was first reported by TMZ.com.With Reporting by BLAINE ZUCKERMAN and HOWARD BREUER