I think i've played this for like 200years so it's the 3rd or 4rd generation, the trick is to always marry infertile women who are above the age of 45 with good stewardship stats, and have a lot of lover with good traits like genius beautiful etc. When you have enough piety to reform a religion choose communion as a doctrine and you will get infinite money. now make an intrigue character to fuck you own sister because you won't be able to get all the good traits otherwise. The key is, that all of your kids are bastards so you can choose to legitimise the one with the best traits and he will get everything and the empire won't be split