Well, in my opinion I am quite sure they are related if not full siblings because their mutation is so unusual and so similar. They are not, however, from the same clutch. They are at least two months apart. Stormborn is growing in her adult feathers. She will become more yellow in the coming weeks, and she has already lost her baby head bars. Her color seems richer and bolder, because it is. Dragon will come to look like her almost exactly in a few months, but for now is more silver because her yellowface mutation has not stained as much of her body. Their adult underfeathers will be cream colored from the YFM2. Their baby feathers are white. Obviously you can see how this will effect the grey tones above.
If pressed, I would guess that they have the same parents who were double clutched, hence their extreme similarity in appearance but difference in age.
The only difference is that Dragon is not a Greywing. She is an Opaline Grey factor, probably masking sky blue- also with YFM2. Grey wing and Grey factor are different you see: grey wing affects the traditionally black striping on the wing feathers ect. Grey factor on the other hand turns the blue body feathers grey. Am I making sense? I think not. >.<