here is one of several great articles on line breeding call ducks by Daphne Mays.... We have personally tried the brother sister mating several times but our fertility drops every time among other things. There are very strong opinions on this subject so you just have to research and try it out for yourself to make a decision.
http://www.longshadowfarms.com/linebreeding.html
http://www.longshadowfarms.com/linebreeding.html