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ATLANTA -- It's one of the mysteries of family - how two girls born of the same parents can be so different.

Olivia is quiet, shy.

Big sister Elena isn't. She speaks her mind. Sitting in her hospital bedin the AFLAC Cancer Center at Children's Healthcare of Atlanta, Elena looks at her nurse Andy before sharing, "Me and Andy had a day where I just cried on her."

Andy says to Elena, "Crying's good. Right?"

Thirteen-year-old Elena is battling a relapse of leukemia.

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Her mom Christy says, "You thought it was bad the first time." She begins to cry. "Cancer is a bad word. It's hard. It's life changing. But relapse is even worse. The battle is harder, the treatment, it's all harder."

But there is some good news; they have a match for a bone marrow transplant. The family has reason to hope.

Dad Mark says, "It's so good to see them, everything they've gone through, so close."

For all their differences, these sisters know the battle the other one faces.

Ten-year-old Olivia sits on the couch with her mother looking at a homemade photographic timeline.

"The third picture was when I was two and I had my first brain tumor," she says.

Olivia has had a brain tumor, twice. First, as a toddler, then it came back when she was six. Today, she is OK. But now her big sister is going away for the transplant needed to save her life.

They will be separated for months because Olivia is too young to visit during the process.

From her hospital bed Elena, cries. "And I don't know if she knows what's going on. It makes me feel like she's kind of left out. And it's also hard because I always hear my parents talking about how they have to switch; one has to be with my sister and one has to be with me. And it's really hard."

Cancer has made these sisters a special, impenetrable pair.

Mark says, "It's almost like we are left out of those feelings, and we're trying to help them the best we can, knowing they know more than we do about what they're dealing with."

Facing a relapse, a transplant, has made13-year-oldElena ask questions no child should have to ask.

"Even sometimes I wonder, is this a punishment from God or something I've done? Or why did God do this? I've wondered that before, and my parents are always like it's not," she says.

But her parents have asked God -- have begged that their girls be delivered from this.

Mark says, "I've prayed that prayer, over and over. Just let it be me. What has she done. Why her? Why them?"

With the separation looming, we got our Help Desk involved to plan something special for this family.

Mark shared with us that Elena "has always said she wanted to be a marine biologist since she was little, and loves to swim, loves the water. That's why this summer is tough on her because she has a central line. She can't get it wet. She can't swim."

When the team at Georgia Aquarium heard about the Elena's dream, they decided to make it come true.

Help Desk reporter Bill Liss met the family at the aquarium and gave them the amazing news that they weren't there for a regular visit. "You're going to join the dolphins. How's that?"

Elena laughs and leans back against her dad. "That's cool. That's really cool."

The Tates got a behind-the-scenes private tour, seats to the dolphin show, and then Elena and Olivia spent serious quality time with two new friends -- and their head trainer Michael Hunt, who showed them how to train and feed the remarkable animals.

It was a fun, laughter and smile filled, hands-on day for sisters who deserve more.

How much is too much? It's something Elena and Olivia and their parents have wondered. What they don't question is their love for each other, the hope that tomorrow will be better.

Christy says, "Do we get angry? Absolutely. Do we get mad? Do we cry? Absolutely. Do we say it's not fair? Sometimes every day. But we can't stay there. It would be so easy to get stuck. In the anger, in the disappointment. But we choose happiness. We choose joy. Every morning I wake up and I thank God for this day, and every night I go to bed and I do the same thing. I thank God for this day."

Use this form to post your wishes for the Tate sisters, soElena can read themin her hospital room and Olivia can read them at home.