It's just, how is inheritance supposed to work? If the inheritance is just you keep marrying people, then there's no real concept of a crown prince ever, there's children of the marriage who go marry other people, they aren't heirs. Heirs are people who marry into the marriage. If the only way for children to inherit is to kill their parents's entire marriage, that sure is something. So Yuri wouldn't ever think he could inherit that, or maybe he's Yuri because he does go "well, murdering them all sounds worth it". But would it then be necessary for Yuri not to be married already? Because presumbly kids marry into other families and joins their inheritance line. I guess they could divorce or split off if someone wants to go inherit something else. But you'd eventually have a lot of people in marriages if they did have to marry all the kids into existing marriages for them to be able to inherit anything, so probably there's some gifts and stuff given to adult children? Marriages would accumulate a lot of stuff, so giving it to the kids is one way to do resource management.

This does explain Xav marrying a Betan, he's starting his own marriage stem. ;)