Robin Roberts' sister reflects on sibling's upcoming wedding after lifesaving bone marrow donation

On Sept. 20, 2012, "Good Morning America" co-anchor Robin Roberts underwent a lifesaving bone marrow transplant using stem cells from her sister, Sally-Ann Roberts, who was her perfect match.

Now, just over one decade later, Sally-Ann Roberts will be by her younger sister's side as Robin Roberts marries her longtime love, Amber Laign.

"When Robin and Amber are saying their vows together, I will just be saying ... 'Thank you Lord, thank you Lord for what you have done for these two,'" Sally-Ann Roberts told "GMA." "Because both of them have faced medical challenges, and both of them have been strong for one another, and so I am just seeing this as the icing on the cake."

Only months before the Roberts sisters underwent the lifesaving bone marrow transplant, Robin Roberts announced to "GMA" viewers that she had been diagnosed with myelodysplastic syndrome, a type of cancer in which immature blood cells in the bone marrow don't mature into healthy blood cells and overcrowd the bone marrow, according to the National Cancer Institute.

One of the treatments for the condition is a bone marrow transplant. After undergoing many tests, Sally-Ann Roberts' stem cells proved to be the perfect match for her younger sister.

After the successful transplant, Sally-Ann Roberts stayed by her sister's side during her months-long recovery, and witnessed the loving care that Laign also provided to her sister.

"I've seen the way Amber loves Robin. I've seen the way Robin loves Amber. Their love is unconditional," she said. "It's a rare thing to find a truly lasting love."

In addition to Robin Roberts' battle with myelodysplastic syndrome, she and Laign have also seen each other through two bouts of breast cancer, with Robin's battle happening early on in their relationship, and Amber's battle happening more recently. The couple of 18 years has also helped one another through personal losses, like the deaths of Roberts' parents, and Laign's father.

Sally-Ann Roberts, one of Robin's three siblings, said she believes the ups and downs of their relationship have created a foundation for an exceptionally strong marriage.

"They have been through the storms of life together. They've had beautiful sunny days together. But I think that a relationship is really tested when the rug appears to be pulled out from under you and yet, with love by your side, you can find a way to stand," she said. "They've been through the good times and bad times together, and I am sure that they are going to have a marvelous future."

Sally-Ann Roberts described Laign as a beloved member of the family already and someone who carries with her a "spirit of adventure, wonder and always love and care."

As she looks ahead to her sister and Laign's upcoming wedding, Sally-Ann Roberts said she feels "so blessed" to be able to see them make a "lifelong commitment" to each other.

"Robin and Amber, you found each other," she said. "Cherish your love, hold on to each other and just know that the best is always yet to come."