To me it sounds like the question is "Do I prefer to polyship OUT to everyone or in the closet?"
And if you prefer to be out in the open about things and included in family life... "Maybe I prefer to polyship in a different way or model than I have here?"
The question of "Am I really monoamorous or polyamorous?" doesn't matter right now in this because you are IN a polyship configuration with limits you do not enjoy. The whole "being out and included in family functions" thing.
You could be monoamorous and love only him, but willing to be in a V situation because you are poly-friendly.
And... the problem still doesn't go away. Because your "loving-ness capacity" is not the problem.
"This polyship's outness and my inclusion in family things I want to be included in" seems to be the issue.
My 2 cents,