Christopher Bonilla has not seen his brother and sister for more than 14 years.

Bonilla, a 30-year-old from the Castle Hill section of the Bronx, was separated from his sister Vanessa, who would now be 22, and his brother Alex, who would now be 17, when he left foster care as a teenager to return to his mother. His younger brother and sister, meanwhile, were adopted by their foster parent.

The last time Bonilla saw them he was about 15 years old. "My mother was getting her [parental] rights terminated and I was in court with her," Bonilla recalled. Chris had a sympathetic social worker who allowed him to return to his mother after he aged out of a group placement and threatened to run away if not returned to her. But Alex and Vanessa were much younger, under the supervision of a separate social worker, and family reunification and supportive services were not then as much of a priority for the Administration for Children's Services.

Bonilla is typical of New Yorkers who were separated from siblings while in foster care and who lost contact with them after adoptions from the foster care system -- which came with financial incentives -- took place.

No one knows how many there are. While ACS data from fiscal year 2014 shows that the "sibling placement rate" was 88.2%, placing and keeping sibling groups together was a less common phenomenon in the past.

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ACS now encourages "adoptive families to consider and support open adoption so that families may maintain contact after an adoption is finalized," said a spokesman for the agency.

Because the records of legally adopted children are sealed, "we encourage adoptees to register with the adoption information registry if they are interested in locating biological family," he said.

But registering is often an exercise in futility. Since the New York State Adoption Information Registry was established in 1983, 41,000 people have signed up, but only 2,405 matches have been made.

"I don't know anyone who has ever located a sibling through a state registry," said Lynn Price, a former foster child who was separated from her own sister and founded "Camp to Belong," to allow foster children to spend time together and cement sibling bonds.

"Facebook has been the most successful," matching up former foster children searching for blood relations, Price said.