Certainly couldn't hurt to see the siblings. You would expect if its an incomplete dominant, ~half of them should have what it is, so it could be recessive. It doesn't look like any of the recessives I'm familiar with. I do see something on MM called "Patternless" but I have no idea how that would behave with Pinstripe, looks pretty new and expensive but you never know what's floating around in someone's lines.