The context is:
She never tired of listening to people even if it demanded endless patience from her.
There were times when the rest of the people in the courtroom would begin to get fidgety, but not a single crease could be seen on Lakshmibai's forehead, who sat erect and bright-eyed.
Given the context, I would have written it differently:
There were times when the rest of the people in the courtroom would begin to get fidgety, but not a single crease could be seen on Lakshmibai's forehead, and she sat erect and bright-eyed.
OR
There were times when the rest of the people in the courtroom would begin to get fidgety, but Lakshmibai, on whose forehead not a single crease could be seen, sat erect and bright-eyed.
Having said that, I don't think it's ungrammatical -- or even unnatural -- as is.
Having an antecedent of a relative word not adjacent to the relative word is a dime a dozen. It alone cannot be the reason for marking it ungrammatical.
It's only when the context is such that having an antecedent not adjacent to the relative word makes it hard to figure out what the real antecedent is.
Here, it is easily understood that the antecedent of 'who' is Lakshmibai, not her forehead, considering:
(1) You don't use 'who' to refer to someone's forehead, unless you want to make it look like a human, which you don't in this context.
(2) A forehead could 'sit erect' but it could never 'sit bright-eyed' unless you have a really peculiar context, which you don't here.