How to stop INCEST !!

Many years ago I had an extensive Steel family and I tried to get two first cousins to date. The game simply wouldn't let me. After that I stuck to marrying my sims outside the family. In the UK marriage between first cousins is permitted certainly in civil (and therefore legal) ceremonies. I concluded therefore that as most of Western Europe does not permit marriage between first cousins and in many states of the USA it is illegal, EA had decided not to allow it in game. I am therefore wondering if there is a glitch in the OP's game ? If not it may be a case of keeping a sheet of paper with the full family tree so you can check for yourself that the relationship is appropriate according to the way you were brought up. No doubt some clever person could come up with a database to do the same thing. Alternatively you could always marry outside the family and that way you are keeping the gene pool fresh just as you would in real life.