you are James' younger sister, rather, James' half sister, but James hates you because you are slytherin

You are two years younger than James, you are Regulus' best friend.

They were in the common room studying, reading quietly among Slytherins. Specially since your mother is a fellow potioneer.

Severus was wanting to improve potions to win over Lily or restore their friendship.

Severus however has a plan to make James furious.

So Severus approaches you

"You are wrong, you have to add the distant at the end"