The sleeves for the 12 Hour dress are really simple - big rectangles with an s-curve cut into the top. They probably should be a little narrower than they are, as cut there are 2 pleats just back of the top shoulder, which does not seem to be a typical style for women's sleeves of the period (1410 or thereabouts). Whatever, it's not unheard of and it hangs well, which is really the main thing.
As far as sources, here are two. There are more, but these are the on-line ones that I used:
April from the Les Tres Riches Heures du duc de Berry :
There is another very similar sleeve, but with dagged edges, in Bibl. de l’Arsenal, ms. 664. fol 47. which I cannot find a picture of to link to. There is a picture of it here on page 7.