Tell them to inspect the party of the brother.
I have had this happend to me in terms of the issue with lords being put into other lords party(there is a few threads on this topic).
In my case one of my parties was rushing to the aid of a city controlled by another kingdome, when I went to my clan-members party, I noticed he had a mercenarylord in his party.
The mercenary lord was hired by that other kingdome.
Like this post :
https://forums.taleworlds.com/index...ve-them-as-party-members.445472/#post-9739019
(Why is that one resolved though, I know a work around but this issue havent been resolved in the latest hotfix, nor can I see any feedback from a dev stateing the issue being resolved either)
This also still happends for other lords, and those can only be fixed if they disband typically by them being defeated.
Still happends in 1.62 Beta for me and I started a new fresh game after the latest hotfix you added the other day.