This is just bizarre. Unless by "the adventure" we mean "GM's story time".
To recap:
@Remathilis posits that "the adventure" requires the PCs to meet a sage who reads Draconic. He then expresses concern that, if the players posit a family member who can help them out (eg read the Draconic), they will bypass or circumvent the plot of the adventure: "if the adventure funnels all choices to B, adding a family member that gets to B is fine, but adding one that avoids B is not."
I then ask, Well why can't the GM just have B show up? I mean, it's the GM who has decided that this sage who speaks Draconic matters, and the GM has a lot of control over the background setting and fiction. So if the GM thinks the sage is so important, why not just narrate a scene in which the sage is there? I mean, this could be anything from the sage visiting the PCs' family member for dinner (Gandalf seems to make a habit of that), to meeting the sage on the road, to having the sage take shelter from a storm at the same inn as the PCs, to . . . etc..
It's not like having B shows up gives the players an "unfair" advantage - the adventure was funnelling all choices to B!, so the players were always meant to find B. It's not like having B show up will mean everyone has to call the session quits and go home - D&D is an open-ended game, and frankly if B is a sage who speaks Draconic I'm 99% sure the GM has some more material in mind that will follow from the encounter with the sage, which sounds like a transition scene rather than the ultimate climax.
It seems to me that, in this thread, the reasons against having a PCs' relative help the PCs get somewhere, or learn something, or gain an audience, always come back to
because the GM's preconception was that the help, or information, or audience, would be achieved in this other fashion that I already decided on. If that's the sort of game the GM is running - setting as puzzle-box - then why would the players bother with PC connections to it? Because those connections will never be relevant, unless the GM happens to incorporate them into their puzzle.