I think that you raise an important question. Does it make sense that Sophie is based on Tweak in that case? Should people fork from Tweak (even before starting using it :)).
Stef
However, IIRC Andreas has stated at at least one occasion in unequivocally clear words that Tweak is primarily for his own projects (Croquet, Etoys(?)). In other words, he is not going to commit to spend time to work at Tweak for the community (knowing him, he might still do it, but we can't count on it ;-)).
Which means that for the Squeak community at large, Tweak is a take-it-or-leave-it deal. People can send patches and/or enhancements to Andreas, but he is likely to decide on accepting them only in the context of his work, and not primarily to do the community a favor.
(note: I'm totally fine with Andreas' position here. It is his code after all, he can do with it as he pleases. Just that people don't misunderstand why I bring up this issue)