There's about three accounts in the Bible the show God's approval of cousincouples as long as it doesn't violate state law. Just find those scriptures and show them to your mom or other family, or whoever ask about your relationship.
Regardless of what they say, remind them that this shows God's view toward cousin couples. If they still insist that it's wrong remind them that, as far as you're concerned, the Bible has the final say so and is the final Authority on all matters of life and it is the Standard that you use to determine what is right and wrong. Then ask them what do they use as a standard for determining right and wrong?
Again, regardless of their answer, no standard is higher or more accurate then God's word.