Four-year-old Michael DeMasi Jr. knows about the needle.

"Yeah, it's a big, giant needle," he said.

He knows it will be plunged into his back on March 8, so doctors at the Children's Hospital of Philadelphia can extract bone marrow, which will then be transplanted to save the lives of his twin 4-month-old brothers, who suffer from a rare immune disease.

DeMasi's parents, Michael DeMasi, 35, and Robin Pownall, 31, of South Philadelphia, call their quiet, shy boy "a superhero" for what he's about to do. But Michael Jr. just considers himself a good big brother.

"I want to help them," he said. "I'm not scared."

DeMasi and Pownall, who have been together for 11 years, have four children: Dominick, 9; Michael Jr., 4; and the 4-month-old fraternal twins: Santino, who goes by "Sonny," and Giovanni, who's called "Gio."

The twins were a surprise, to say the least. The couple wasn't planning on getting pregnant, and when they found out they were having twins, it was a curveball.

"God's a good pitcher, I'll tell you that," Pownall said. "He threw us a good one."

It's not the first curveball the family's been thrown. The couple's first son, Dominick, was diagnosed with a rare immune disease called Chronic Granulomatous Disease (CGD) and spent nearly his entire first year of life at CHOP before an umbilical cord donation from a stranger gave him the stem cell transplant he needed to survive.

According to Pownall, who's had to become an armchair expert on CGD, it is a disease that affects the immune system, making it impossible for the body to fight infection.

"It's like the bubble-boy syndrome," she said. "If we don't do a transplant, they can get very sick from a small infection. A cut can be fatal."

But eight years after his transplant, Dominick is a healthy, high-energy, fast-talking kid. He is, essentially, cured.

CGD is hereditary, and Pownall found out she was a carrier. Only one in 500,000 people is diagnosed with CGD every year and it tends to affect boys at a higher rate than girls, she said.

Michael Jr. wasn't born with the disease, but when Pownall found out she was pregnant with twins, she feared the worst. The twins were born on Oct. 14 at 33 weeks, and spent the first five weeks of their lives at the neonatal intensive care unit of a local hospital. They were home for only 10 days, during which time they tested positive for CGD, and Santino developed an infection from his circumcision.

Both boys were then taken to CHOP, where they've lived since November. Pownall sleeps next to them in a cot in their room. She must wear a face mask and gown at all times when with her baby boys, so she doesn't pass along any infections.

"I have to sneak kisses," she said. "They're my babies. How am I not going to kiss them?"

>> Read more: Pennsylvania doesn't really know how many babies are born dependent on opioids. That's a big problem.

The family knew a transplant was the only way to give the twins a shot at a normal life. A perfect sibling bone marrow transplant match is even better than an umbilical cord stem cell match, Pownall said, so when Michael Jr. tested as a match for both boys, his parents asked him if he wanted to donate his bone marrow. They explained to him what he would have to do.

"We wanted to give him that option. People think he doesn't realize what's happening, but he knows what's going on and he's on board," she said. "He's so brave. He is our courageous boy."

Citing hospital privacy rules, a spokeswoman for CHOP declined to offer any comment on the case.

On Monday, the twins — who only weigh 11 and 12 pounds — will begin 10 days of chemotherapy before the transplant.

"They wipe their immune systems entirely and then give them the bone marrow," Pownall said. "It's a very serious, scary operation for all three, actually."

Michael DeMasi Sr., the boys' father, said he tries to keep a clear mind in order to cope.

"I try not to think about it," he said. "There is that risk they could not make it, and I don't want to think about that."

Pownall, whose positive and calm demeanor belie her emotional and physical exhaustion, said she has moments where the reality of what's happening crashes upon her.

"Sometimes I just break down. I'll be in the shower, and it will just all come out," she said. "And then I'll meet a mom at CHOP whose daughter just got diagnosed with stage-four cancer and it brings me back, and I remember how grateful I am that there is hope for my situation."

If the transplant goes well, Michael Jr. will be able to leave the hospital the same day as the operation, and the twins should be able to head home four-to-six weeks later.

When they are reunited, their first wish is a simple one — to go camping and fishing together as family.

"I'm so grateful for all the support we've received. I'm so grateful CHOP is right here," Pownall said. "And I'm so grateful that this can be a positive story."

 A GoFundMe page has been has been set up to help the family with expenses.