Mikayla Stern-Ellis, 19, and Emily Nappi, 18, met at Tulane University in New Orleans when they became roommates via a Facebook ad.
As they chatted about their pasts, they both found they had been conceived by a donation from an anonymous donor.
Neither thought much of it, even when they figured out that both of their donors were Colombian.
When the girls went home for winter break, Mikayla told her parents about her new friend.
Her mum Heidi then looked through the donor list from two decades ago, and while there were some 1,000 donors on the list, couldn't find any others with Colombian ethnicity.
Then after seeing pictures of Emily, and remarking on how much she looked like her own daughter, she told Mikayla to ask Emily for her sperm donor father's number.
When the numbers matched, the girls were surprised (to say the least) – although they had joked to their college friends that there was a '25 per cent chance we're sisters'.
Following the discovery, the girls' similarities now make more sense. They're both interested in drama and dedicated followers of similar tastes in fashion.
For Mikayla, finding her half-sister in a college friend is proof that she's at the right school.
The two have since changed their Facebook profile pictures to an image of them hugging.
After posting the picture on her profile, Mikayla wrote: "An invisible thread connects those who are destined to meet, regardless of time, place, and circumstance."