I tend to think in terms of “we” – in terms of “one for each.”  And yet in the murk of sibling rivalry, I first size up the bigger half, and decide if I want it.  Oh, yes, there is such a thing – a bigger half.  Split a sandwich between any non-identical twins under the age of 8, and ask them.  It’s always obvious which is the bigger half; and the curse of getting the smaller half is in no way cured by the triumph of the receiving the larger half.

“She got more than me.”  It doesn’t matter who says it.
“Give your sister a bite of your sandwich,”  Mother says, reaching toward your plate to even the score.  Or worse:
“You can have an extra cookie.  Now eat your lunch.”

At this point, feelings may simmer, or explode.  There is no justice in this world, as your permanent, undetachable other self (who is always stealing what you want the most) continues munching what should be YOUR sandwich – or tries to grab YOUR extra cookie for herself.  Or, most cruelly of all, eludes the conflict altogether, shrugging off her tinier portion with an escape artists’ precision, spoiling the taste of all that extra peanut butter and jelly with her indifference.