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It’s no secret that twins share a special bond. But Dawn Ardolino Policastro has proved how deep that bond can be: She carried her twin sister’s baby when she was unable to.

The New York resident and mother of two served as a gestational surrogate for her identical twin sister, Allison Ardolino Dinkelacker, Patch reports. (A gestational surrogate carries a couple’s fertilized egg.)

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Allison discovered she had stage 3 breast cancer in 2009 when she was 30 weeks pregnant with her first son, Dylan. She had to start chemotherapy immediately but was able to deliver Dylan via an emergency C-section at 31 weeks.

“When Allison was diagnosed at such a young age and at a time in her life when she was the happiest, it was simply devastating,” Dawn told Patch.

Allison’s cancer has been in remission for nearly six years, but since her cancer was brought on by hormones, she can never get pregnant again.

Dawn, who lives just a few blocks away from Allison, was at an oncologist’s appointment with Allison when she was told that she would never be able to carry another child.

“Well before he even finished his sentence, Dawn jumped in and said, ‘It doesn’t matter because I am going to carry their child,’” Allison wrote on her Facebook page, per Patch.

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The twins posed for photos together during the pregnancy for photographer Allison Rose, who has known them since they were little.

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Dawn, who has two sons, gave birth to a healthy baby boy, Hudson William Dinkelacker, on August 5. (She kept the baby’s gender a secret from her sister during the pregnancy.)

Allison’s Facebook message to her sister after Hudson’s birth will make you tear up:

“When I was diagnosed with breast cancer six years ago I knew that me being told I had cancer was as devastating to Dawn as it was to me. After all, we have always seen it as we are one soul in two different bodies,” Allison wrote, per the Allison Rose’s Facebook page. “How can we begin to thank you for the tremendous generosity and sacrifice you have so willingly bestowed these last few months? You have given us not just the fulfillment of a wish we’ve had for the last six years, but a whole new life, and a family of four we thought we’d never have.”