Why is my sister so selfish?

Im 28(f) .. Sister is 26(f).

We both live at home with our mother and I make a significant contribution to the household bills and grocery shop. My sister is a medical school student but is also working fulltime in a psychiatric ward as a PA. She earns good money.

As mentioned before, I do the food shop 90% of the time and she struggles to make a contribution. I even pay for my mother's holiday and most of the holiday to Santorini for my sister last year for her birthday.

Last night, she raged at me for eating an Apple that she purchased through an M&S voucher that was given to her. But.. I did a whole weekly shop at M&S for the whole family and everything I buy - she finished it.
She raged at me calling me names and swearing profusely, shouting down the house because of this.

My mother tried to reason with her but she would not stop raging. I don't understand this irrational behaviour and where it stems from. It had greatly destroyed the relationship I thought I had with her cos I look after these women. Wholeheartedly. But its not being reciprocated.

Could someone please help me understand.

Thanks

Im 28(f) .. Sister is 26(f).

We both live at home with our mother and I make a significant contribution to the household bills and grocery shop. My sister is a medical school student but is also working fulltime in a psychiatric ward as a PA. She earns good money.

As mentioned before, I do the food shop 90% of the time and she struggles to make a contribution. I even pay for my mother's holiday and most of the holiday to Santorini for my sister last year for her birthday.

Last night, she raged at me for eating an Apple that she purchased through an M&S voucher that was given to her. But.. I did a whole weekly shop at M&S for the whole family and everything I buy - she finished it.
She raged at me calling me names and swearing profusely, shouting down the house because of this.

My mother tried to reason with her but she would not stop raging. I don't understand this irrational behaviour and where it stems from. It had greatly destroyed the relationship I thought I had with her cos I look after these women. Wholeheartedly. But its not being reciprocated.

Could someone please help me understand.

Thanks