The way that I understand triangulation, is that if there is conflict between person A and B, one or both of them pull in a third person to help them deal with the discomfort caused by the other. There is often an attempt on A or B's part to enlist C to help them deal with the other.

In churches, and example of this would be "Betty" gets upset at the pastor. Instead of talking to the pastor, Betty talks to "Fred", and wants Fred to go talk to the pastor and get the pastor to stop doing whatever she is upset about. Sometimes, this works for all parties. Betty gets what she needs, Fred is helpful, Betty doesn't have to endure facing the pastor herself, pastor gets feedback from a more neutral person that Betty which can make it easier to hear and accommodate.

But, sometimes this just increases tension all around. Fred takes on Betty's feelings, feels stress because he has to address the pastor, the attempt to intervene results in tension between Fred and Pastor, Pastor feels like people are ganging up on him, Betty doesn't have the opportunity to deal with her own feelings in a healthy way, Fred may or may not accurately address Betty's concerns, Pastor and Betty may feel weird around each other. Etc.

From what I have read about your family dynamic, the tension may exist between D and H, and you sometimes get pulled into the middle. If that is the case, the "primary" relationship would be between D and H for the purposes of understanding the triangle.