Hi Quackmaster,
There is a chart on the forum about dirt deficiencies where one of the cases looks like that -Veins darker in the leaf - from memory iron deficiency - don't quote me on that one :) .
It could well be lack of sun . Is that stem heading North and shaded by her sister ?
If so try to gently bend the sister towards the dirt to allow more sun to the odd looking one - "gently" like in try to not break it.
Did you fertilize that tree this year already ? If not, time to do so . At least, it will help the stems grow and reach out for the sun.
I would check as well around for rodents. There could be rodent activity on that side of the tree and the roots got cut, thus leaving that side with less food .
Good luck !