Constantine's position in the imperial family were changed in 326 when Constantine I poisoned Crispus and steam boiled Fausta in her bathroom by bolting the door from the outside. Constantine arranged this when he received a message that the two had been spied on having sex. Pre-Christianity, this wouldn't have been much of a deal - emperor Claudius had married his niece and Caligula had learnt all about sex from his sisters. However this was now the Christian era and the church was very against any suggestions of siblings bonking each other.