The adoptive parents of Sherin Mathews have given up their fight for custody of their biological 4-year-old daughter.

In a custody hearing Friday morning, Wesley and Sini Mathews stood in shackles before a judge and agreed to relinquish their parental rights — an irrevocable decision.  

Wesley Mathews has been charged with capital murder in 3-year-old Sherin's October death. Sini Mathews was arrested on a charge of child abandonment or endangerment based on her husband's admission to investigators that they went out to dinner and left Sherin alone the night before she died. 

Sini Mathews "made the extremely difficult decision to give up her parental rights because, given the circumstances and the pending criminal cases, this is in the best interests of the child. She wants what's best for her remaining daughter," said attorney Mitch Nolte who represents the mother. 

Last month a Dallas judge blocked Sherin Mathews' parents from having contact with their surviving child after prosecutors argued that they failed to protect Sherin. The judge's ruling meant that Child Protective Services didn't have to work with the Richardson couple to reunite them with the 4-year-old girl. When the agency removes children from their homes, it typically offers the parents services such as parenting classes and counseling to help them regain custody of their kids.

CPS has custody of Sherin's sister, who has been temporarily placed with relatives in the Houston area.

Those relatives, who went through extensive background checks and home studies, plan to adopt the girl, said Marissa Gonzales, a spokeswoman for the Texas Department of Family Protective Services. 

Authorities have said Sherin was killed by "homicidal violence" before her adoptive father hid her body in a Richardson culvert where it went undiscovered for weeks.  

The Dallas County capital murder indictment against the father says he killed Sherin "by a manner and means unknown to the grand jury," court records show.

If convicted, Mathews could face the death penalty, should prosecutors choose to pursue it, or an automatic sentence of life without parole.

Mathews, 37, was arrested in October on a felony injury to a child charge after the girl's body was found Oct. 22. He originally said he put her outside Oct. 7 because she wouldn't drink her milk. He later changed that story to say that the child choked on her milk. 

He is charged with four felonies, including injury to a child and capital murder. He is also charged with abandoning a child and tampering with physical evidence.

Investigators have not said specifically how Sherin was killed, and her body had been outside for weeks before she was found and an autopsy could be performed. A doctor found multiple healing fractures on the child in March. 

Sherin's disappearance and death attracted worldwide attention and fascination to a little girl whose family appeared to favor their biological daughter to Sherin, who was adopted from India in 2016. The family's Richardson house had many photos of their biological daughter but none of Sherin. 

The Indian government suspended the U.S. adoption agency involved in placing Sherin with the Mathews family. 

The girls' mother, Sini Mathews, remains in jail, with bail set at $250,000.