Yes, I know what you mean. I just kept thinking of the rooster because it did set in when he was older and there were many similarities between your hen and him.I've seen those.
I think my bird has something genetic related, rather then Auto Immune Disease related. All of the things I've read on vitiligo say it effects birds of a few months of age.
Previous posts I've made includes the parents. I can link them here for you, or just repost them.
I've seen the photos of the parents and I do agree that the white on the mother's primaries does look very similar to the way the white is spreading on your hen. It does make me lean towards something genetic as well. I was just throwing those past thread out as another idea, but it will definitely take some time and test breeding to figure out what is really going on.