I am glad I stumbled onto this poem.
You have written this at a time in your life when any previous confusion of Joan's actions become clear. You describe the annoyance from her having to share what was for 4 years hers. I can appreciate this, being the eldest of 3. Like you though, once we become aware, we then realise how it was for them. I know now that they and you too only wanted acceptance from your older sibling. I admit I did not appreciate this at the time.
My sister passed some years ago now, and I wish I'd told her more often how much she meant to me. I think of her daily and I'm sure Joan thinks of you too.