I think this form will be taken care by the siblings of theymos. As with all the business, their ownership is never transferred to the external shareholders.
What makes you think he even has siblings? Unless he wrote about it somewhere on the forum. Considering how private of a person theymos is, I doubt he did. I don't think Bitcointalk is something those close to theymos will inherit. He didn't inherit it by being the child of the previous admin either. Passing it on to children or siblings should not be something he would seriously consider unless they have a major interest in bitcoin and forum management, and they are already on the forum without us knowing about it.
To me also the idea that some relatives of theymos inherit the management of this forum does not seem too likely and possible.
After all, theymos was not originally the main admin (but only the global moderator) of this forum, and after the departure of Satoshi and some other admins, there was no vacuum here, but theymos took over the management of the forum.
In the same way, if and when theymos leave this forum someone other from the current staff will naturally replace him.
There is no need to worry too much about this issue.