Some decisions are taken or rules are made on the basis of different communities and particular places of residence. Your relationship is not restricted to marriage, but still, it is most important that you get more information about your local rules from your family members or other people of your society. Marriage is an important decision and naturally, you will always want to be with the person you love but family is an important part. As you mentioned that your relationship is already a family relationship, so I don't think you will have much difficulty in getting information. There are many such practices or rules in some societies where marriage can take place in this type of family relationship. As far as legal permission is concerned, when the customs or rules of the community have been going on for a long time, they are recognized for law. Agreeing is an important part of the family, so your first love should be in the direction of how your loved ones view and adopt this relationship.