What was the first plane that allowed walking during flight? What was the first plane that allowed crew members and/or passengers to stand and walk during flight?

P.S. I mean walking inside the cabin, as part of normal operation intended for by the plane designers, not walking outside the plane, on wings and other acrobatic activity.
The first airplane with a cabin big enough for walking around was indeed a Sikorsky. This was the Sikorsky Russky Vityas, also sometimes named "Le Grand", a name which was given to an earlier, two-engined version. Oh, and the cabin included a washroom, too!



Sikorsky Russky Vityas (picture source)

It was the first four-engined airplane in the world and the biggest airplane of its time. It flew first on May 10 (or 13, depending on the source), 1913.

Quote from Wikipedia:


  After the Russky Vityaz's first test flights between 10 and 27 May
  1913 (Old Style dates), it was established that a passenger could even
  walk around the cabins without causing any problems to stability.




Early Le Grand version with two engines (source)