@Christyfrank Yes, they are planning a sort of fundraiser so that she can take center stage. She needs the support of the nobility to become her brother's legal guardian. As things are, they probably wouldn't give it to her, specially since she is not yet married and therefore not yet a noble.
She can't make any obvious movements to gain that support. A ball is therefore the perfect social event during which she can start gaining influence without ruffling noble feathers. Throwing the first ball after two years under the pretext of a fundraiser would enable her to start growing a reputation other than "the Emperor's commoner sister", as a member of the Black Sun and as a cultured woman.