3) Cardinal already knocked up Ariadne's mother in between his children , it shows how lecherous he was didn't care if a girl was consenting or not.
So Lucretia was married to cardinal while he was made aware he got her pregnant with Ippolito which was actually from her Lover. Cardinal was fooled into thinking it's his child. Maybe it was just a tiny bump no more different than a bloated stomach. About Isabelle maybe Lucretia cheated again on him (deserved).
Lucretia's family must've forced him to marry her that's why he has a hate boner for her family. He has some idea about Isabella maybe but not Ippolito or who knows maybe cause he doesn't have an heir so he just accepted it.
As suspected, it seems Isabella also isnt the cardinals child, but this brings up so many questions...
1) why didnt Ariadne question her about this? She kept skipping Isabella, it's very obvious
2) ur life is nothing, u were already going to die
3) how was she already heavily, or at least noticeably pregnant and able to marry a CARDINAL??? How has he not realised timeline wise the baby couldn't be his?