"All parents/childs/sibling" is somewhat vague. Perhaps you could rephrase to be more descriptive. Where do you want to start from? Which direction do you want to go? How far do you want to go? When searching down, would you rather go as deep as you can first, searching from one side of the tree to the other or row by row?

Parenting forms a tree.

Searching only this GameObject's parents (going up the tree):

var foo : Transform;
var current : Transform = transform;
while(current.parent) {
    current = current.parent;
    if(current.name == "foo") {
        foo = current;
        break;
    }
}

Searching all immediate children (searching the next row of the tree can be achieved by Transform.Find:

var foo = transform.Find("foo");

To search from the highest parent (the top of the tree), you would need to get this root transform and do the above there like so:

var foo = transform.root.Find("foo");

This is more meaningful if you know the exact path which you would delimit with '/' and grandchildren will be searched.

To search only siblings, you would go up one parent and then check all of its children like so:

var foo : Transform;
if(transform.parent) foo = transform.parent.Find("foo");

It isn't as fast, but to search all children at every level, you would use Transform.GetComponentsInChildren like so:

var foo : Transform;
for(var child : Transform in transform.GetComponentsInChildren(Transform)) {
    if(child.name == "foo") {
        foo = child;
        break;
    }
}

Likewise, to search the entire tree with no knowledge of the relationships, you would do as above with the root:

var foo : Transform;
for(var child : Transform in transform.root.GetComponentsInChildren(Transform)) {
    if(child.name == "foo") {
        foo = child;
        break;
    }
}

Using GetComponentsInChildren uses Depth-First-Search. If you had wanted to do a Breadth-First-Search, you would have to write one like so:

var foo : Transform;
var queue : Array = new Array();
for(var child : Transform in transform) queue.push(child);
while(queue.length != 0) {
    var current = queue.shift();
    if(current.name == "foo") {
        foo = current;
        break;
    }
    for(var child : Transform in current) queue.push(child);
}  

It's already built into the engine. You need to grab all the children from the parent by following along here:

http://unity3d.com/support/documentation/ScriptReference/GameObject.GetComponentsInChildren.html

The code you will want to modify to check each object for a tag. (I'm assuming you want an alternative to FindGameObjectByTag

var transforms : Transform[];
transforms = gameObject.GetComponentsInChildren(Transform);
for (var transform : Transform in transforms) {
    if(transform.gameobject.tag == "target") {
      //Do what you like.
    }
}

Looking at your addendum, I think what you want to do is get the parent of the objects you want to search, then cycle through all children via the GetComponentsInChildren above until you get the GameObject you are looking for.

http://unity3d.com/support/documentation/ScriptReference/index.Accessing_Other_Game_Objects.html

Check number 2 please.

Should be good =).