I don´t think you even need to marry that princess to anyone, and if you´re lucky that ol´geezer of yours dies without leaving an heir.
It´s rather small change of getting Inbred for 1 son or daughter... In real life it actually took hundreds of years until there were totally mad and deformed members of the royal family.
I had level 2 inbred dude once... He got married to normal noble woman and he was fertile... Otherwise nice guy expects he had 2 of 4 sons with level 2 inbred, other two had none expects they both were drunkard maniacs...