A: In modern usage, “behalf” is an invariable noun and has no plural form. The old plural “behalves” is considered obsolete and has been for some time. It's labeled “obsolete,” for example, in my 1956 copy of Webster's New International Dictionary (the unabridged second edition).

Is on our Behalves correct?

If your friend and his siblings wanted to use such an expression, they should have written “on our behalf” instead of “on all of our behalves.

How do you use Behalves?

In behalf, they argue, is used when the meaning is in the interest of someone else, but on behalf is used when speaking for someone. For example, a medical decision would be made in behalf of the patient, and you would speak on behalf of your family. The reality is that on behalf is typically used for both meanings.