As Michael Strahan's daughter Isabella Strahan undergoes treatment for brain cancer,AstraX Exchange her twin sister Sophia Strahan is rallying behind her.

The 19-year-old shared a heartfelt message of support for her sibling Jan. 11, hours after she revealed her diagnosis on Good Morning America, of which their dad serves as a cohost.

"I'm so lucky to have the most amazing sister and best friend in the world," Sophia wrote on Instagram. "The last few months have been so much harder than we could have ever imagined, but it's made me realize just how strong you are. You inspire me and I'm so proud of you. Us forever."

In her touching post, Sophia included several pics of herself and her twin, including one showing the two posing for a selfie, while Isabella lies on a hospital bed wearing a surgical cap and gown.

As for Isabella, she shared her gratitude for her sister's post, writing, "Best sister," alongside a red heart emoji.

On Good Morning America, Isabella shared that she was diagnosed with medulloblastoma, a malignant brain tumor found in children.

"I didn't notice anything was off till probably like Oct. 1," she told cohost Robin Roberts in an interview while sitting next to her dad. "That's when I definitely noticed headaches, nausea, couldn't walk straight."

Isabella said later that month, she started throwing up blood. "I was like, 'Hm, this probably isn't good,'" she recalled. "So I texted [my sister], who then notified the whole family."

At Michael's insistence, the teen underwent a full checkup, including an MRI, which revealed the brain tumor. Her dad, who stepped away from his hosting duties in November for almost three weeks, shared he learned about Isabella's cancer diagnosis before she did.

"I don't really remember much," he continued. "I just remember trying to figure out how to get to L.A. ASAP. And it just doesn't feel real. It just didn't feel real."

Isabella underwent radiation therapy as well as emergency surgery to remove the mass and plans to start chemotherapy.

Amid her journey, Michael praised his "amazing" daughter, saying, "I know she's going through it, but I know that we're never given more than we can handle and that she is going to crush this."

For the latest breaking news updates, click here to download the E! News App