My response would be this:
This is primarily an internal guild thing, but since it does touch up against the anti-muling rules, I'd say it would require some consideration of that perspective.
You have said this new character is the brother of a founder who has since died. The key question is, at any time prior to the death of said founder was the new character ever names as an heir or successor in any way. Making such declarations after-the-fact are not really valid.
For example, if this brother character was created after the first character died, then I'd definitely say "no," because it just looks too much like an attempt to mule and to gain an advantage (i.e. inheritance, right of birth, etc.). Things like this are expressly forbidden for character creation, I don't see why such a claim would be considered after-the-fact. I use this same metric for deciding on wills in the Grievance forum.
On the other hand, if the brother character was around and listed as a beneficiary prior to the death of the first character, then at at least it can be argued and scrutinized.
Even so, my personal opinion is that the "shares" of the guild should be divided between the surviving founding members. To do otherwise runs the risk of crossing that thin grey line.
