Guys I don't think they are related. I thought that at first but Assisi's mother was the mistress of Caitel's after Assisi and Caitel were born. Assisi and Ria aren't related although everyone else seems to be
This situation is quite funny. ( ̄∇ ̄") In most isekais, people tend to be pissed off that the reincarnated person is going after a younger person. However, in this case, it's the opposite. However, I'm not going to disagree to the fact she didn't act much like an adult, which was a big contributing factor, and that they do feel like a guardian/babysitter relationship.

guys... it seems that y'all have forgotten.... Assisi is Caitels HALF BROTHER! ! !
how could u ship him with ria