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It looks like any family's Sunday dinner: 4-year-old big sister Bobbie, curly-haired and precocious, sings to her cooing baby sister Esther while the parents laugh at their antics. But around the table in this cozy home in Tacoma, Wash., there are not two adults but four: Jodi and Larry Dillworth, the biological parents of both girls, and Rachel and Diony Victorin, who adopted now-14-month-old Esther as an embryo left over from the Dillworths' in vitro fertilization. The Dillworths, of Tacoma, and the Victorins, who live in nearby Kent, have agreed to raise Bobbie and Esther as sisters in different homes. "This," says Larry, "is the ultimate gift of love."

It's a gift with some strings attached. Since the practice started over a decade ago, most of the roughly 3,500 babies born through embryo donation have had no contact with their biological parents. But in the past few years, dozens of embryo-donor couples have worked out both informal and written agreements with adoptive parents to allow for everything from occasional visits to an ongoing relationship with their biological child. "Children have a need to know where they came from, and having that history hidden can create feelings of dissatisfaction," says Dr. Jeffrey Keenan, director of the nonprofit National Embryo Donation Center. "Ideally, every embryo could be donated openly." So far, there have been no publicized disputes between couples, but with the practice gaining in popularity and some 600,000 frozen embryos stored in labs around the country, some experts say it's just a matter of time before problems crop up. Open embryo-adoption arrangements "can confuse a child," says Dr. Curtis E. Harris, a medical-ethics professor and lawyer who has been involved in drafting more than 50 embryo-adoption agreements. "An attachment [to the biological parents] can occur that shouldn't be occurring."

For their part, the Dillworths and Victorins think the more love surrounding their kids the better-especially after the heartache each couple endured trying to create a family. After conceiving through IVF in 2007, Jodi, 41, had five leftover embryos; medical problems led to a hysterectomy. Knowing she couldn't carry more pregnancies, she still "couldn't bear to donate [the embryos] to research." She found Embryo Adoption Services of Cedar Park, a nonprofit embryo adoption agency. Reading profiles of prospective parents, she came across the Victorins, desperate for a child after four failed in vitro attempts. "I knew I was meant to be a mother," says Rachel, 38, a nurse.

Her dream came true Sept. 28, 2011, when she gave birth to 6-lb., 5-oz. Esther Hope. The couples had signed a standard agreement providing for the Dillworths to receive an annual update, but the families decided they wanted more contact after Rachel and Diony, 40, a graduate student, reached out. Three months later they met, girls in tow, at a fast-food restaurant. Jodi wasn't sure she was ready: "I was a nervous wreck." Larry worried, "Am I going to want to grab my child and take off? I didn't know." When Rachel placed Esther into Larry's arms, he started to tear up. "She felt like Bobbie as a baby," he says. "After a few minutes, it seemed natural."

It's also pretty simple, for now. Getting the girls together as often as once a week for meals and playdates, the Dillworths have told Bobbie that Esther "is a special part of our family," Jodi says. Rachel says they'll make it clear to Esther they are her only parents, but that the Dillworths "will play a part in her life, and her life will be richer knowing them." Yet both couples know challenges lie ahead. "Right now we are all very happy," Larry says. "Talk to us in 20 years." One thing is certain: Two little people seem perfectly content. At Esther's first birthday party, Bobbie watched as the toddler played with an alphabet train, a gift from big sister. "She is very pretty," Bobbie said, putting her finger into Esther's tiny fist. "I want to name my baby doll Esther, just like her."

BIOLOGICAL MOM Jodi Dillworth

BIOLOGICAL SISTER Bobbie Dillworth

ADOPTIVE DAD Diony Victorin

BABY ESTHER at 8 months

BIOLOGICAL DAD Larry Dillworth

BIRTH/ADOPTIVE MOM Rachel Victorin