Although the main reason for night blindness is believed to be the lack of vitamin A in the body, you have not been told that since when your fiance's father has this disease, actually due to lack of vitamins in the body, there is weakness in the eyes. And there is less in the eyesight, in some cases, it causes night blindness in the eyes. In some other cases, there is also a problem of night blindness due to cataracts. There are also some other factors like lack of diet.
There is not some mainly factual example for night blindness in genetic form, but it would not be right to deny it completely. In this case, it is better that you consult a good ophthalmologist. Along with this, when you and your family will take equal care of your health, that is, give importance to a balanced diet, then this type of problem is less likely to happen in the future.
Marriage is an important and special part of life, in which the happiness of family along with us is considered not only for the immediate but also for the future. For this reason, it is appropriate for your family to be concerned. You should consult a good eye specialist and clear all doubts of you and your family. Night blindness is not such a disease that can not be treated, so just relax and be concerned with a specialist.