Cousin's Sister Abnormal By Birth. Any Chances Of Hard Luck If I Marry My Cousin?

Hello!! i am very glad to have found this. My Question is: I m 21. My cousin s sister (my mother s sister s children) was abnormal by birth. acted like a 2 years old child, couldn t walk due to bad organs, till her death in age of 15, nearly. There are some other abnormalities in her paternal family. no abnormality in our parent s generation. I want to marry my cousin, is their any chance that we could have such hard luck in children?? one more thing, my paternal family and my family is extraordinarily intelligent and technical, but have one apparent fault. We have loose temperament. Even, my uncle had to quit his professional medical studies when he got caught in a mental disease . now, he is just normal, except for, he has very very loose temperament, gets mad at small things, 52 years, unmarried. Please help me find my answer.