The issue is not on biped, but on what you use to skin your mesh. Are you using physique? Or the skin modifier?
They are both great, although I never use physique, because I never use Character Studio: I prefer to go on traditional bone-skin rigging because (as I have previously discussed here at the forums) it is more professional and provides greater control.
Now, about envelopes, there is a good chance that envelopes work very similar in physique and skin, so I am going to tell you what might be happening to you.
Normally these issues happen due to envelopes overlapping (if I understood you right). It can not be avoided at first. Actually, it is quite impossible to have a well deformed mesh just by setting the envelopes right. You often have to go into a second step consisting on refining vertex weights, specially for the joints vertexes. That means to fine tune how those vertexes are influenced by any surrounding bones. As you do that, you will notice how some or many of your vertex are being carried by several bones at the same time (due to initial envelope overlapping) thus resulting in unexpected behavior. You then use the available tools to fix that (if you are using skin you use the weight table, the paint weight tools and manual weight assignment).
With the weight tools you can adjust how much any bone influences any vertex (or set of vertexs). I am sure physique has something like that. So, never forget that step for correct skining. It might take long to set up, but you will end having a very nice skinning (yet not perfect… you would need the third step: angle morphing).
And, here is a very good advice for to tune up your skin faster:s
Setting your envelopes or weights and after that trying the result by moving you model could result in a very long work session. Instead you should try the following. Make a simple animation of your fingers bending, starting with the initial pose on frame 0. So, as you scroll the time bar, your finger will bend, thus showing the mesh deformation. You can even adjust your envelopes/weight at a frame where your fingers are bent (no animating-autokey off) so you see right away the effect it has over the deformed mesh. When you get the desired deformation, then you can erase your animation, and there you go.
Sorry about my English.
Good luck with your fingers!
