I agee with ren, but that leads to the same effect, as the sibling line will always be same element as the Hexagram.
In that sense if we look at what is controling the Sibling line, that would already be the Officer/Ghost line.
I meant that the self line can also show the persons depending on the question. And the Self line can be anything. And there will be something attacking/controlling it.
In the previous example:

The Self line is Metal. That is combining with the Si line, but the Si line is also controlling it and its also somewhat moving in the way The Monk read it.
So here the idea is that from Si controlling the Self line we again get Image of Officer/Control relation, so that Si line can also show illness, even though its resource.
It makes different way of reading it, as once we have Officer from what attacks the hexagram element, that is what attacks Sibling as well(as Sibling will always be Hexagram element). But then we also have similar images in the line that still controls the Self Line.
In this example, Self line is child. Resource is controlling it(as resource does). So from the point of view of the Self line, Fire is the Officer.
Meaning if we make the relation based on different points of view, we get different line as officer. And in this case, Self lines Officer is fire and that is resource.
In this sense there are more then a few things we can look for illness. Officer line - sure. Line that turns to Officer - sure. White Tiger in some situations I guess. But then we also have the line that is controlling the Self line or the other persons line.
Also that can happen if we ask for someone else. Lets say we ask for a parent. Parent line may represent them, but then the line that will attack them is the Wealth line. In that case Child won't be the cure depending on what Officer line we watch, as the Child line will be producing the illness line. Suggesting the medicine that the person is using may be making everything worse.
Same way as in bazi if we want to see another spouse or boyfriend for female in bazi, but we need to do it in a males chart(lets say his girlfriend) we can look at her element from her point of view. So for Wealth, Officer line will be SIbling(as Sibling controls Wealth). Again Officer relation/images, but shown indirectly. I think its called "shifting palaces" there or whatever, but the general idea that we can make the relation based on the line that we find interesting to follow.
We have more then a few examples with that, as some members in the other forums used only that. The illness was always the line that attacks the Self line(or the patients line).
In my view, I agree with Tientai and Ren that other stuff is what we should first look for illness(the initial Officer line, as that will not only control the Sibling line, but also the Hexagram element as a whole), but I do think the line that is Officer in relation to the Self line also can play a role and in other situations other lines as well.