i'll have to try very hard not to sway you. to give you some details: of her two brothers, he's the one who sees calpurnia's worth. after she was kidnapped when she was younger, it came out to him (either via the king or one of her informants, haven't decided yet) that she's something of the royal family's spymaster and had been putting down political enemies and pre-empting threats against ostwill for several years at that point-- half to stick it to the family for thinking she was worthless, half because she'd do anything to preserve the kingdom/peace.
fortunately she was such a huge pain in the ass that she was deemed not worth the trouble and dumped back on the palace doorstep before the ransom was delivered. but it showed the need to have a decoy. she both understands and resents the crap out of it. woo, classic pissy calpurnia conundrums and awkward family dynamics. so basically all the rumors of the king having a sneaky political powerhouse kid were transferred on to him, while calpurnia continues to sit around in the background looking bored and malcontent and imagining everyone spontaneously combusting so she can get back to her dresses and embroidering and lounging luxuriously on her many chaises.