I think that at 6, it's too early to mention this. It's kind of irrelevant to her life right now, and might only induce longing that you wont be able to fulfill.
The best time to tell her, IMO, is either when it comes up naturally in conversation -- the two of you are discussing the subject of bio-dad and siblings, and you say, "I think he has X number of kids, their names are X, etc.", or sometime after she is about 10.
Whatever the case, I think the subject should evolve from her curiosity, but it's not something she just HAS to know about at this point in time.
When it comes up, it should be explored as something positive -- I wonder what they are like, maybe you will meet them some day, etc. -- not like she has missed out by not knowing about them until now.
Keep the conversation light, when you have it. Someday she will probably meet them, and she might love them, or she might never want to see them again as long as she lives. Either way is fine.
p.s. You've handled everything perfectly about bio-dad until now -- I think you will know what to do with this issue. :)