To date, there are seventeen volumes in the True Tenchi Muyo! GXP series of light novels. Volume 10 was written by Masaki Kajishima to augment the events of episodes 18 and 19 in the anime. It is unavailable outside of Japan.
In no short order, the vacation at the Masaki estate turns to terror. Washu in her mischief has turned Tenchi's grandfather into a ten year-old. She sets about calling him "Yosho-kun". Soon, relatives and guests throughout the house fuss over and pamper the new kiddo.
Washu does not stop there as she likewise transforms Airi into her preteen form. Everyone is aghast with how easily Young Airi gets along with Young Yosho for the rest of the morning.
Ringo sets in motion some overt interrogations between Seina, Neiju, Amane and a few other visitors. Neiju in particular reveals that she is fixed in a larval state unlike Washu. She was purpose-built to play the role of a shrine maiden and, like Kenshi, develop powers.
Seina breaks from followup discussions with Ryoko Balta only to get overwhelmed by Amane's sudden proximity. She reminds him that they are close enough to informally discuss bedtime. He thinks she means to talk about things as if among a friend of the same sex.
But as she continues to emphasize that she is an older woman, he stresses that even with their distance from the Academy on this holiday, Seiryo would surely appear and break it up. He asks Amane to take a rain check.
Seina wanders about lamenting that all the time on vacation could have been spent training.
Sure enough, Seiryo meets with the Kamidake crew in space while in command of a ship flying the flag of Daluma's Good Luck Fleet. They set about attacking each other, but the Kamidake must escape to hyperspace before long.
Seto inspects the damage to the Kamidake once it has safely docked and is quite spooked.