The Ragtag Daily Prompt this morning is CHANGELING
Merriam-Webster gives these definitions:
Turncoat (archaic)
One child exchanged for another at birth, (usually a fairy child)
Imbecile (archaic)
And I’m going to go with the first meaning, though it be rather archaic.

No Changing Allowed!
Sister stamped her foot in fury. “Changeling! Turncoat! Traitor to the cause!”
“I’ve seen the light,” Brother responded. “It’s not an issue.”
“You were on my side before. Now you’re wimping out.”
“Having given the matter serious thought, I’ve realized that one choice is as good as the other.” Brother maintained his calm tone. “It’s no big deal.”
“Ha! If you’d been in Boston before the Tea Party, you’d probably have said ‘It’s no big deal. Let’s just pay the tax and not rock the boat’.”
“It might have saved a war.”
“Heretic!” Sister punched his arm. “And this is a big deal!”
“Everybody raves about how great peace is. ‘NO MORE WAR,’ they say. But soon as they get passionate about some issue, they’re ready to take up arms. Like you now.”
“You must have been switched for my real brother at the hospital. If you were my true brother you’d see things like I do.”
Brother scowled. “Wow! Talk about over-reacting.”
“Somebody needs to remind you of what you said last month when this issue first came up. You’ve done a 180 switch.”
“All I said was, I think we should…”
“But you said just the opposite last month. You agreed with me then. Changeling. Traitor.”
Finally Dad spoke up. “Okay, you two. Rather than fighting about this – ”
“I’m not fighting about it,” Brother protested. “She is.”
“I’m not fighting, either! I’m just saying he can’t change his mind like this. Last month we decided we wanted to do Sea World. Now he’s saying let’s go to Yellowstone.”
Dad laid his hand on her shoulder. “Well, I’ll settle the matter. We’ll visit Yellowstone this summer and Sea World next year. End of the skirmish. And no sniping.”
“Who knows if I’ll even be alive next year? The whole world may lay in ashes!”
Dad frowned at her. “No Sniping. End of subject.”