If things get rocky between any pair in the relationship, there is nearly always some sort of impact on the third - it's hard to avoid, really. Sometimes it's just feeling the empathy of what the other two are going through, sometimes it is being a little more flexible to give them room to get things sorted. It might even be a role of catalyst/facilitator to get the two to talk about stuff openly, if there is some breakdown in communication. That latter I could imagine could be a tad tricky, and would need to be handled carefully.
This is very similar to how I view things, though recently I've discovered that sometimes the issues between other people in my family do affect me more directly.
While nothing like the example from the OP, I do find myself struggling a bit with the feelings I have because I was impacted by a situation I was not directly involved in. I'm trying to figure out how best to work on preventing any of us from feeling the same way in the future but navigating uncharted waters needs to be a slow and careful process.
It was interesting to come here today and see the subject of this thread; even though the situation doesn't directly correlate, it was good to see others navigating those same waters.