Exactly! Was her brother her real brother? If he was, that's proof enough that not all of the family has that power. And if it were even somewhat common among their family, would they not test their children and she would have already known she had that power all along?

But her brother wasn't immune to fire/heat, clearly, so doesn't that mean that not every person in their lineage has the fire immune power? that it's just something specific to her? If their entire family had that ability, you would think it would be common knowledge, or a spoken story/legend.

The waaaaaaiiittinnng is the haaaaardest part.