I am a Pentecostal who is surrounded by a family of Catholics. Why, if Jesus stated "except a man be born again he can not enter the kingdom of heaven" do they not believe in being born again?
The proper people to ask why they don’t believe in something would be the people themselves. That is, to find out why your relatives don’t believe in being born again you would have to ask them. I suspect that you would be told that they do believe in being born again.
Most Catholics I know believe firmly that “except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.” (John 3:5) Water baptism (although what they call baptism is not necessarily the immersion of the Bible) is one of the basic sacraments of the Roman Catholic Church. It is a basic tenet, as I understand Catholic doctrine, that one must be born again, and that one must be born of water (baptism, the external part of the sacrament) and the Spirit (the internal part of the sacrament). Most Catholics would disagree with anyone who said they did not believe in being born again. If your relatives do not believe in that part of Catholic doctrine, they would be the ones who could answer why not.
Of course, their definition of being born again, which closely parallels the Bible’s description of being born again (the only objection I have being the baptism of infants), may not agree with your definition of being born again. It may be that you don’t think they believe in being born again because you have a different concept of what that phrase means. If so, you would need to define terms first, and then ask the question.