"He's only weighing nine ounces and his sister is over two pounds, so big size difference," Brittani said.
"His only chance of survival would be heart surgery, but they wouldn't do heart surgery on him because of his brain," she added.
Tuesday, they received a glimmer of hope thanks to a sonogram image.
The technician told the couple to look at the image, their unborn son was holding his bigger sister, Madilyn's, hand, KWCH reported.
"Most twins, when she's trying to take pictures and stuff, they're kicking each other and hitting each other and she said with our twins it seems like she was very protective over him," Brittani said.
Doctors do not expect Mason to survive, but his parents know he will be protected as long as he lives.
"I know I'm holding him, I'm carrying him, but I just want to be there for him. And she's the only one who can actually be there and holding onto him through it, so it's comforting to know that if he does pass he won't be alone," Brittani said.