I also think LW needs to start gently teaching the son the basics of self-control. It's not okay to "throw a fit" if someone else's birthday cake doesn't meet your specifications. I mean, definitely get him a cupcake so he has a treat and isn't left out, but it's not doing the kid any favors to let everything in the family constantly revolve around him (and not just, I suspect, when it's a matter of food.) I have the impression--and this may be unfair, because the letter brought back bad memories for me--that LW is constantly prioritizing the son and expecting/instructing the daughter to accept that she gets less attention to her needs, wants, comfort.

I was the older "sister" (I'm a trans man, but didn't know it then) of a difficult, demanding younger brother who always got his way, while I was expected to be patient and mature and understanding, and I am STILL BITTER.

It also strikes me as a very gendered expectation. If the daughter were an older brother instead, would LW still expect him to be "easygoing" about not even getting the birthday treat he wanted?