Marrying your cousin may not be accepted in your community and you will be putting your families in a humiliating position if they will allow your marriage. Think about what your children will be facing in the future: genetic defects and disapproval or discrimination from other people.
not mention how wrong that is, but in nature, animals tend not to mate with siblings, because of recessive genetic diseases, that are usually lethal.
I too heard about this genetic defect in the children born from siblings, cousins, or to anyone related by first degree. Also, in some cultures marrying within the family is not allowed because it is considered immorality. If this is allowed in your country then there shouldn't be any problem about getting permission from your parents. But you will be bringing a lot of health risks to your future children. Just think about the pros and cons before making a decision.
Google 'cousins's marriage' and similar website to understand the problems you will face in marrying your cousin sister. Also consider the relatioship current as well as future ones. Marrying your children will also be one of the problem in future.I am sure you must have given deep consideration to all probable problems. Consider whether you will be able to face all the problems.Genetic defect can multiply if cousins get married.Think before you commit.