Leah Messer has always had one of the saddest stories on Teen Mom, if not the saddest.

And that's because one of her children suffers from a very serious illness.

She earned her place on Teen Mom 2 after giving birth to twin daughters she named Aleeah and Aliannah, who now go by Gracie and Ali.

In the early seasons of the show, we saw her notice that one of the girls, Ali, wasn't developing at the same rate as her sister.

Leah has said that even when the girls were born, she and Corey Simms noticed that something wasn't quite right with her legs.

As the twins' first birthday approached, they saw that Ali's legs weren't strong enough to hold her weight so that she could walk, and she started working hard to get some answers.

After seeing many doctors and getting lots of opinions, they finally got an answer when she was three years old: she had a rare form of muscular dystrophy.

In fact, she was the first child to ever be diagnosed with that particular type, so doctors aren't really sure how the disease is going to progress.

As the years have gone on, we've seen Leah take Ali to regular doctor appointments where her doctor urges her to use her wheelchair and wear a helmet.

We've also seen Ali struggle with not being able to do the same things her sister Gracie does, and Gracie struggle with Ali needing so much extra attention.

Thankfully the family seems to be doing much better these days with how they're dealing with everything, but unfortunately muscular dystrophy is a progressive disease.

Still, Leah has been doing everything in her power to give Ali an awesome childhood, and she offered a great example of that on Instagram.

If you keep up with Leah, you probably know that Gracie is big into cheerleading -- she does competitions, and the whole family comes along.

Ali has expressed interest in cheerleading as well, but she's just not physically capable of doing the same kinds of things that Gracie does.

And that's why Leah did this very sweet thing for her last week.

In her Instagram post, she wrote that "If you are ever wondering where I find my hope and determination, it is in this girl!", along with a photo of Ali.

"There was a day last week that Ali had a rough day, simply trying to understand why she has muscular dystrophy," she continued.

"She didn't comprehend why her sisters were reaching milestones that she wasn't able to. She just wanted to do something that brought her joy and made her feel included."

She explained that "She just wanted to be able to ride a bike and run with her sisters. She wanted to be able to do these things without it being too hard and getting hurt."

That's so hard to hear, but the good news is that this story has a happy ending.

"Today she was smiling from ear to ear as her sisters cheered her on during a one on one private," meaning that she had a private session with a cheer coach.

She thanked Gracie's cheerleading team "for allowing my girl to feel a part of the family."

"Ali's hope and determination through all circumstances always inspire me," she wrote. "I'm so blessed to be her momma."

"I'm seriously so proud of her, and I know she felt so much joy leaving the gym today. Her smile and joy melted my heart."

She finished her post by telling Ali that she loves her so much, and saying "Today seriously meant so much to my Ali girl! Thank you!"

Leah hasn't always made the best parenting choices, but it's clear she truly loves her daughters, and that she's doing her best in these impossibly tough circumstances.

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We hope that Ali keeps having great days like this one!