Best aunt ever?

Photos of two Long Island twin sisters, one embracing the other's pregnant belly, have gone viral on the Internet this week, because these aren't your typical maternity session portraits.

In one of the images shot by professional photographer Allison Maffettone of Westbury, the sisters -- Allison Ardolino Dinkelacker and Dawn Ardolino Policastro of Mineola -- hold a chalkboard sign that reads "My bun ..." with an arrow pointing to Dinkelacker, and "her oven," with an arrow aimed at Policastro's baby bump.

That's because Policastro was carrying her sister's baby. It was the only way Dinkelacker could grow her family, Maffettone explained on her Facebook page, Allison Rose Photography.

In 2009, Dinkelacker, then 32 years old and 30 weeks pregnant with her first child, was diagnosed with stage 3 breast cancer, Maffettone wrote while posting the photos on July 28.

"It was so aggressive that she had to start chemotherapy immediately," Maffettone wrote. "The day after her first treatment, she went for a sonogram and the baby wasn't moving."

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Dinkelacker had an emergency C-section at 31 weeks, on the sisters' 33rd birthday, but was able to deliver a healthy baby boy, whom she named Dylan, before going on to have six months of chemotherapy, 35 rounds of radiation and numerous surgeries, Maffettone said in her post.

But, she added, since the cancer was "hormone positive," Dinkelacker would be unable to carry another baby herself.

Policastro agreed to be a surrogate, and she delivered her nephew, Hudson William Dinkelacker, on Aug. 5, according to a separate Facebook post by Maffettone. (Policastro also kept the baby's gender a secret from her sister throughout the pregnancy.)

In the post, Maffettone quoted Dinkelacker, who called her sister Policastro "her hero," and opened up about her fight with cancer.

"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," Dinkelacker said in the post. "After all we have always seen it as we are one soul in two different bodies."

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Dinkelacker also alluded to some of the struggles the sisters encountered in their attempt to carry a baby this way.

"When most people would have given up during the struggles we endured, you just pushed through it all," she said. "Time and time again when we thought we should quit, you never gave up."

The photos of the sisters on Maffettone's Facebook page have received more than 80,000 "Likes" and have been shared by more than 15,000 Facebook users as of Thursday morning.

"We cannot thank you enough for the gift you have given us," Dinkelacker wrote to her sister. "Although biologically this child will be made up of the two of us, we certainly hope that the strength, selflessness, and power you have shown are traits that he takes from you during his life."

Policastro, when reached via email, said she and her sister were not available for interviews.