"You know, people ask comedians, 'Where does the funny come from?' And, for me, it comes from my desire to never have anyone feel what I felt through my childhood and certain periods of my life," UsMagazine.com quoted Underwood as saying.
"So, I try to make everyone laugh and everyone happy," she said. "I was born a twin, and we were premature. We were both in the incubator. What you don't know is I carry her birth certificate around because she did not live."
Underwood went on to say her mother used the baby's death to distance the future comedian from her father.
"My birth mother did not want me and my father to be close to each other, so she would tell me that my father killed my sister," Underwood said, adding her mother suggested she had asphyxiated when he "propped a bottle up for her to take."
"For years -- and I love my father -- but I did not get very close to him until one day, I saw my mother stab my father," she said. "A lot of people have domestic violence in their families. A lot of people witness a lot of things, so to get through that, I started to engage my sister, my twin, as my guardian angel."