I'm not always sure how this even happens; my patients may swear up and down that they have not said anything yet to anyone. But sometimes even relatives that did not seem to be involved at all seem to just come out of the woodwork and get involved.
For siblings and other relatives, the concerns usually center around a fear that the primary target will not be able to handle the confrontation, and may decompensate in some way, or that the confrontation may create tensions in other important dyadic relationships within the family (for example, between the parents). Sometimes, a sibling may fear having to step into a family role previously played by the metacommunicator.
They also admit to the triangulator that they themselves have had similar concerns. Even though they may have felt the same exact way in the past, however, metacommunicators often become extremely annoyed with the relative for having any negative attitude towards the plan. As difficult as it may be to muster, an empathic response based on identifying the triangulator's feelings in oneself is far more effective in getting the triangular to keep out of it.
The last sentence is designed to reduce the likelihood that the potential triangulator will go ahead and interfere! Family systems folk call this a paradoxical request. The statement appeals to the triangulator for cooperation while indicating that the patient will not be drawn into a power struggle about it.