The Gossip Girl star is suing Constance Meester amid claims she took her daughter's monthly payments of $7,500 (£4,700), intended to take care of her brother Alexander, who suffers from severe medical issues, and used them for cosmetic enhancements for herself.
Constance is also accused of trying to wring more money from Leighton by declaring they had agreed on an oral contract which required the star to pay her $10,000 (£6,250) every month for life.
The actress wants a judge to declare that no such agreement exists, but Constance hit back with a countersuit on Monday (25Jul11), insisting the promise of financial security was in exchange for the sacrifices her mum had already made to turn Leighton into a star.
Constance has gone even further, alleging her daughter attacked her during a fall out in December (10), reports TMZ.com.
The legal papers state, "Leighton intentionally assaulted Constance by throwing her to the ground and repeatedly hitting her with a bottle."
And she's now claiming the actress' actions are harming the welfare of young Alexander.
The countersuit continues, "Since Alexander receives 24 hour care from Constance, no alternative source of income is possible. As a direct result of Leighton's breach, there are no funds to feed and care for Alexander."