@Elendin
There's nothing wrong with having never celebrated a birthday - I, for one, have never celebrated my own or others. Having never celebrated a birthday doesn't say anything about their parents.
The issue here is the favouritism and massive bias that the parents have towards her twin - which is an inevitable occurrence in any and all relationships, just that the contrast here is larger and apparently for no reason (there are supposedly reasons that have yet to be shown in the scanlation of the manga).