The grandparents, who live on King's Road in Greenville, have permanent custody of the children. Neighbors said the kids are 7 and 9 years old.
Deputies said they began their investigation on Feb. 4, after the grandparents found out about the alleged plot. According to authorities, the children told them their mother had mailed a knife to be used as the murder weapon.
The grandparents told NewsChannel 12 they found the blade, similar to a steak knife, under the pillow of one of the girls. They said they contacted authorities after they made the discovery.
A neighbor, who did not want to be identified, said the children would never have carried out the alleged plot.
"They are the sweetest little girls and I don't think they'd hurt anybody," the neighbor said. "The grandparents are doing a wonderful job raising them."
The suspect, Leticia Silva, refers to herself as "Lethal" on her Facebook page.