If Amara had the power all along, and she only waited until all three of her sons were together in one place that still doesn’t explain how she was going to make Jafar do the spell with her.
Well she seemed pretty sure she was stronger than him. Heck. She almost killed him if he hadn’t killed Cyrus, I think she would have won. I think perhaps, she just let Jafar gather her sons, and would have let him cast the spell at which point she would have returned to normal, and then disposed of Jafar.
"I could have the giant duck as my steed!" --Daniel Radcliffe
Keeper Of Tamara's Taser , Jafar's Staff, Kitsis’s Glasses , Ariel’s Tail, Dopey's Hat , Peter Pan’s Shadow, Outfit, & Pied Cloak,Red Queen's Castle, White Rabbit's Power To World Hop, Zelena's BroomStick, & ALL MAGIC