We Need to Have A Talk About Writing...

This is a lovely, light-hearted story. It put a smile on my face. Your characters are very likable and the loved the bond between Seya and Jonnz. Nothing like a little sibling rivalry to get the balls rolling. I did notice a few errors, but they're simple really. Nothing a little editing can't fix.

Love the title. Also loved the opening line. Nice buildup of suspense when their father returned. I was eager to find out what had them in such awe.

Below, I have placed some suggestions in brackets.

"Stop eyeing my basket and see about those eggs," my sister(,)Seya(,) said. "Before Nanny Cook starts wondering what else she can think of to keep us busy."
(I would replace the period after said with a comma to merge the sentence as the second half of the dialogue cannot stand alone as a sentence.)
For example: "Stop eyeing my basket and see about those eggs," my sister, Seya, said, "before Nanny Cook starts wondering what else she can think of to keep us busy."

"It doesn't matter anyway, Father will be back from the yard sales soon," I grumped.
(I wouldn't use grumped in this way as it's an action as opposed to a tagline.)

“Well?” Lance exclaimed, his arms wide in an exaggerated gesture. “Well(,) Seya? Well(,) Jonnz?”
(I added the commas as you are directly addressing the characters.)