Sasami and Ryoko make an interesting pair. Although the Japanese convention seems to be for anyone of Sasami's kind of age to refer to anyone of Ryoko's kind of age as big sister "oneechan", Sasami seems to take this to the ultimate level. Not only does she call Ryoko "oneechan", but she treats the pirate as if she were a second sister...potentially another reason for Ayeka to so dislike the pirate, we must assume!
Sasami's mischievous streak and Ryoko's wild edge often seem in harmony with one another. The first time we meet Sasami in the OVA, she's daring Tenchi to break into Ayeka's room and steal her headdress, just because she's bored. She seems unconcerned by the fact that Tenchi is Ayeka's prisoner and she seems amused by the consequences...and when all things are considered, this comes over as a very Ryoko thing to do. It's a sort of irony that in some ways Ryoko is the one to blame (or so it appears) for Sasami's near death experience and her assimilation with Tsunami. (Although it's probably this connection existed beforehand because of Sasami's appearance...it's not really known for sure). Perhaps this day in their mutual history has forged a bond between them, or perhaps Sasami is simply not the kind of girl to hold a grudge. Either way, she has a genuine affection for Ryoko - although she is not above playing tricks on the girl if she thinks she can get away with it. It's Sasami, after all, who first puts into Ryoko's head the idea that Noboyuki's books are really guides on how to fall in love!
In Tenchi Universe, the sister ethic is even more in evidence. Sasami is as keen as Ryoko to get involved in potentially dangerous adventures - including the exploration of Washu's cave - and on occasion even takes Ryoko's side against Ayeka, when her sister is making some damning criticism of Ryoko's behaviour. One such example is in No Need For Swimsuits, when Ayeka makes a snide remark about Ryoko not doing anything, and Sasami points out that Ayeka hasn't been doing much, either. When they are stranded at the checkpoint, Ryoko is genuinely worried about Sasami when she gets seperated from the pack - another indication of the pseudo-sister bond between them. Ryoko is also unconcerned by Sasami's bond with Ryo Ohki - who is, in effect, Ryoko's spaceship and form of transport as well as her friend.
Part of the relationship could simply be that Sasami is a fabulous cook and Ryoko has a healthy appetite, but it seems to be more than that. When Sasami disappears aboard the ghost ship, Ryoko and Ayeka are willing to team up with one another to help find her, showing their mutual concern for the little girl.
In Manatsu No Eve, Ryoko and Sasami have a slightly more turbulent relationship. Sasami is very friendly with Mayuka, who Ryoko distrusts, and at one point Sasami accuses Ryoko of being mean, because she doesn't want to try and help Mayuka escape the dark spell Yuzuha has on her. And yet, it is Ryoko and Tenchi who plunge into Yuzuha's world to save Sasami - and when Ryoko is injured, Sasami tries to help her up. Sasami's fit of pique, it seems, was simply surface frustration after all!
Despite the occasional prank playing and the subtle hints that Sasami might well be interested in Tenchi, Ryoko doesn't really seem to see her as competition in any of the series. She's surprised by the idea in the OVA, and it's never really pursued in the other shows except on the rare occasion when Sasami manages to get to Tenchi first.