Motoko has always been at odds with men, and when her sister decided to get married, Motoko couldn't handle it and came to Tokyo. But on occasion her sister does visit and is addressed as "sister", even by those other than Motoko. Elegant, beautiful, with an air of mystique in her traditional attire: Tsuruko is truly captivating. But also dangerous. She's said to be the best swordsman of the Shimeiru clan within the last 100 years and even when not totally serious she's powerful enough to cut an oncoming truck in half that would have hit Keitaro. You can imagine what she's like when she's pissed. Needless to say, once Motoko's sister decides that Keitaro and Motoko should get married, some people get very nervous. It's not exactly like you can tell Motoko's sister "no"...
Keitaro last recalls his grandmother running an inn called Hinata Inn. But somewhere along the way, the Inn was converted into a girl's dormitory - which would lead one to believe that Keitaro never really visits his grandmother. In fact you never even see her. In the anime she sets off on some sort of quest to find love, but in the manga she just decides to go on an extended vacation. She must be a rather carefree person in order to simply give over her inn to a grandson that never visits her... But then again there is also some evidence that Hinata Inn was getting to be a bit too much for her to run, as much of it had fallen into disrepair. Proof being in the gaping hole in the floor in Naru's room which conveniently allowed her to talk to grandma Urashima. Of course once Keitaro moved into the room, the hole saw a lot more "action".
Amara is the older sister of Kaolla that only appears in the anime, with exotic looks and a knack for dramatic entrances. Like Kaola, she tends to mix up parts of the Japanese language. For example saying that she has a "Meji" accent (Meji being an era in Japanese history...) Amara is also accompanied by her pet albino alligator, who seems to enjoy chewing on people.
While Nyamo isn't directly related to anyone in the story, but her grandfather was an archaeologist who was a good friend of Seta back during his evil cult fighting days. Assumably he's passed on, and her only keepsake of him is his picture around her neck within a locket. Nyamo is rather unusual in that she looks almost exactly like Shinobu, except for a slightly darker complexion. Nyamo is a very timid girl, and almost never speaks. Not so surprisingly by the point where she turns up in the story, she has a fondness for Keitaro. After seeing repeated attacks by Naru on Keitaro, she tends to fear Naru.
Sara is the somewhat cute daughter of Seta. As is an assistant of Seta, Keitaro tends to end up babysitting Sara a lot. This is bad news for Keitaro, due to the fact that Sara tends to make his life hell, and the pretty much the only time she behaves herself is around her father or when some other plot is being devised. Sara's mother died when she was very young, and as Seta travels all the time she ended up staying at Hinata Inn. Once there, she tends to hang around with Su.