I'm 18 and am pregnant with my second child, am I being irresponsible for keeping it?

Who cares what your cousin says. It is YOUR decision, and your fiance's as well. If you want to have another child and think you can handle it, that is your choice, and your family should support you. If you want to have an abortion, and think that is the best thing for you and your family, that is also your choice. Good luck with whatever you decide!