While things were looking grim for Beatrix, Freya and Steiner in Alexandria, they do make it to safety - thanks to Tantalus members Marcus and Blank. By the time Garnet returns to Alexandria, a lot has changed. Queen Brahne is dead. She met her end on the Outer Continent when she summoned the eidolon Bahamut to kill Kuja - who she felt was no longer any use to her. However, Kuja was able to control the eidolon himself with the power of Invincible – his airship – and he commanded Bahamut to attack Brahne. In her final moments, Brahne seems to have awoken from the mad haze she was in, and reunites with Garnet. She asks for Garnet's forgiveness before she passes away.
By now, Garnet has learned a bit more about herself. Queen Brahne was actually her adoptive mother. Garnet was actually born in Madain Sari, where the summoner tribe lived. The tribe is now all but wiped out, with only a little girl named Eiko left. Eiko lives in Madain Sari with a group of moogles, more or less alone since the death of her grandfather a few years ago.
Garnet has been through a lot, but still manages to remain strong, although she is understandably somber. She visits the tomb of her mother and asks Steiner and Beatrix to help her with the burden of ruling. They both, of course, agree to do so.
Preparations are made for Garnet's ascension to the throne. The troops which had been stationed in Lindblum after Brahne's attack on that city are recalled. After the war and turmoil of recent events, Alexandria is at peace.
Garnet and Steiner have travelled together over the course of the game, and it's shown their closeness and Steiner's devotion to her. We now get to see a glimpse of a similar relationship that Garnet has Beatrix. Beatrix assists Garnet when she is getting changed into her queenly attire. You would think Garnet would have a maid or similar person rather than the general of her kingdom doing that, but it's obviously something that Beatrix doesn't mind doing. She cares greatly for Garnet, and in truth has quite a big heart behind the arrogant and sometimes cold demeanour.
Once she is changed, Garnet informs Beatrix of her true heritage. Beatrix, however, is already aware of the fact that Brahne was not Garnet's birth mother - as is Steiner.
It makes no difference to Beatrix - her loyalty and affection for Garnet is not dependent on her having royal blood. She will be with Garnet until the end, and is a shoulder for Garnet to lean on as she navigates being queen and dealing with the grief of losing her mother.