Being a twin of the same gender must not be too much fun, especially if parents dress you alike. Fortunately, I'm a middle child, so there's no confusing me :)

I like the conflict that you set up.

Even when people knew there were two of them, they preferred to call them both “Jane”. It was difficult to have someone approach her and say “Mary” first. Jane was who they all cared about.

I would be 😤 if that happened to me. People might even do it unconsciously too.

There was something fishy going on. Their mother never prepared their favorite dishes except it was their birthday or she wanted them to do something inconvenient for her. Mary could see that there was also a big roast chicken at the center of the table. That was Jane's favorite.

This is a good way of providing some anticipation for the fireworks to come. It concisely builds up the action, as if taking a deep breath.

Turning to Jane, she said, “As for you, keep out of my business until then and once I leave, keep out of my life”

It's like she had been waiting a long time to say this, and she let pyrotechnics fly.

Mary picked her backpack up and walked out, she didn't even turn around when for the first time, her neighbor said “Good morning Mary” instead of “Good morning Jane”.

This is an excellent ending. It demonstrates that now that she's more assertive, it shows in the way she carries herself and people notice that. Great story!