I'm sorry, I don't have much of any information about the girls' clothes in general or those dresses in particular. All I can tell you that might be of help is that the woman seen here with OTMAA's clothing is Tamara Korshounova, a curator at the AP:

Not sure if she's still there, but it might be worth an inquiry.
Incidentally, this image was posted by Lisa Aubrey in a another thread some years ago:

It's by Brisac, and came from the Alexander Palace in 1917, so could have belonged to OTMA. I don't think it's a match with the dresses in the 1913 or 1914 sessions, but it's similar enough in style and construction that there's reason to hope that more dresses may have survived.
I meant in all three of my posts ONLY the 1914 dresses with beading they wear in the 1914 photos, which I believe they also wear in the 1913 formal photos, not any of their other clothes.
IMO, the dresses are different in the 1913 and 1914 sessions.
In 1913, the Little Pair's dresses had beaded kaftan-like sashes and a cuff of beading around each sleeve that isn't present in the 1914 portraits. You can see the sleeve details
here on Maria's dress. The sash is much more difficult to pick out in B&W, but if you can find a careful colorization of
this 1913 pose, it should be visible along the waist and down the front of Anastasia's dress, running alongside Aleksei's sailor collar.
In 1914, the Big Pair had tassels on their sleeves, and a triangular panel of embroidery on the bodice. The tassels show up nicely in
this 1914 profile of Tatiana, and the embroidery
here, below Tatiana's brooch.
I thought perhaps that in 1914 the Little Pair are just wearing the Big Pair's dresses from 1913, but now I believe they're different. In
this 1913 pose of the Big Pair, you can see that the beaded pattern on the bodice of Olga's gown is circular, with something like an upside down tulip pointing to the waistline. Yet the Little Pair's 1914 gowns have a pattern of horizontal festoons across the bodice -- seen clearly
here.
That said, the Little Pair's 1914 skirts are similar enough to their sisters' 1913 gowns that I wouldn't be surprised if at least part of Maria and Anastasia's dresses in 1914 were in fact made over from the Big Pair's 1913 gowns. So you may be looking for a minimum of SIX dresses:
2 - Little Pair 1913
2 - Big Pair 1913/Little Pair 1914
2 - Big Pair 1914