A stepsibling taking your side against their own parent is a big moment. Notice it.

Choosing you over their own parent costs a stepsibling something real. They live in that house longer than the argument does, and they will be the one at the kitchen table afterwards. When somebody pays that price for you, it is not politeness or a mood. It is a decision about who they count as family.

If that has happened for you, say so plainly, once. A short thank you, not a scene. And if the day comes when their parent is unfair to them, be the one in the room who says it out loud.