May I add my opinion to this matter?
I seem to recall that we have discussed this issue elsewhere, although the location of the discussion escapes me at present.
From the information that we have about the night of the murders and from the knowledge that we have of the IF, it would seem to me that the girls would have been wearing corsets of a fairly conventional nature.
Granted, the brassière had been around since the early years of the twentieth century ( I think that the first patent for this type of garment was issued in 1913); however, IMO the ladies of the IF would have been too conservative in taste and incllination to have worn such garments.
We have testimony that the remains (the busk fronts) of corsets were found at the mine, so it is safe to assume that these were worn all the time that the female members of the IF were dressed.
It is feasible, therefore, that the jewels were sewn between two chemises that were then worn under their corsets. By this time, corsets could be purchased and were not all custom made; whether this was the case or not matters little, as the point of the corset is that the setting of it is variable and that its primary aim is to smooth the line of the figure under the clothing, give an added layer of 'protection' between top and bottom layers, and not simply to compress and contort the figure into ridiculously small waists, (Alix would never have permitted her daughters to wear corsets that were too tightly laced in any case). Thus the corset could be worn tighter or looser, according to taste.
These corsets would have covered the lower bust and the torso, down through the waist and onto the hip, dipping in front. They may also have had shoulder straps.
Someone else has mentioned that their outer garments were quite loose; skirts that fitted only at the waist coupled with loose blouses would have meant that these chemises could be worn under loosened corsets, in order to conceal the jewellery.
The jewellery in question was in the form of packets of loose stones, I believe, and would have been easier to conceal in this fashion than bulky pieces of jewellery in their settings. The wadding that was apparently used as an interlining between the two chemises would have been dual-purpose; to conceal the nature of the hidden elements and to cushion the torsos of the ladies from the intrinsic hardness of the stones.
This method of concealing the jewellery seems to me to be the most consistent with the known mode of dressing of the female members of the IF; chemises with corsets worn on top seems to me the most likely method that they employed.