The general admission floor area is only accessible by guests who have a GA Floor ticket. In this case, general admission simply means that all floor "seats" are un-reserved and floor ticketholders may stand whereever they choose.
Section 323 is located slightly behind the stage. As such, only a limited number of tickets were sold in that section. Because of its location, this section is highly unlikely to go filled. You may be able to purchase a cheap ticket in another section for your in-law with the assumption that she will be able to sit with you.