Vassal Inheritance Warning

Are you sure that their children are their heirs? If they're using a system of inheritance like seniority it could pass to someone outside your Duchy.

I had a similar issue, and I managed to figure out why.

In my game, the Duchy of Calabria is in danger of passing from the Kingdom of Sicily, despite the fact that its heir is the current Duke's son. As it turns out, the heir's mother is the Duchess of the Aegean Isles. Because the Aegean Isles are part of Byzantium, if the Duchess dies before the Duke, the heir will be a Byzantine vassal when he inherits Calabria, so that duchy will leave my realm.

Fortunately for me, the Byzantines have Medium Crown Authority, so if the Duke dies before the Duchess, the Aegean Isles will instead leave Byzantium.

Have you checked to see if there are any other titles involved?

erekose123 said:

As Duke of Saxony, i got this message warning me that the Bishopric of gera might pass from my Realm.
The Bisop Udo of Gera has a Heir, Erpho von Münster, who I married my Daughter matrilineary to. They now have a beautiful son and a daughter, but I still get the message, and i dont understand why.

Click to expand...

I suppose, the problem is that Erpho von Münster is a vassal (or courtier) of somebody else. Even if he inherits something, he'll stay the vassal of that person. To solve this, you could either try to get free investiture and appoint a successor from your own lands, or try to invite Erpho to your court. Or, of course, assassinate him.
Erpho's children don't matter in this case, btw, since bishorics don't have dynasty-based succession.