As said before NEVER add a person twice you are only asking for trouble.
You should begin to NOT think in terms of the # of cousin they are to you, but instead enter the information that relates them to you and the "cousinship" will emerge.
So you must enter all of the people up the tree from person "A" until the point that both person "A's" and "B's" common ancenstor is added, then down the tree to "B". To add "C" you must also do the same thing.
After you have added "B" and "C" then adding the children to this family will automatically make then your cousin. Again you are adding people to the tree (as they are related to their "next of kin") NOT as cousins to youself.
You should begin to NOT think in terms of the # of cousin they are to you, but instead enter the information that relates them to you and the "cousinship" will emerge.
So you must enter all of the people up the tree from person "A" until the point that both person "A's" and "B's" common ancenstor is added, then down the tree to "B". To add "C" you must also do the same thing.
After you have added "B" and "C" then adding the children to this family will automatically make then your cousin. Again you are adding people to the tree (as they are related to their "next of kin") NOT as cousins to youself.