Kai'Sa and Nidus are as similar as Taric and Trinity. (in other words, they have a scant few similarities, but that's about it.)
Sure, they're both some entity (Kai'Sa, the Tenno) controlling a parasitic bio-weapon (the Void, the Infestation) through some form of symbiosis, but the similarities end there.
Kai'Sa is a woman who just barely survived the Void by resisting a parasite and forming something of a symbiotic relationship with it - she hunts other Voidspawn and siphons their energy to literally feed the beast. She uses corrupted, hungering, and esoteric magics to stalk and kill.
Nidus, on the other hand, is an ancient weapon designed by a deified empire for the use of powerful, hyper-lethal warriors. Nidus in particular was designed to emulate a previous bio-weapon of their design, the Infestation. It was designed so that the Tenno could scourge the Orokin's opposition with a rampant, self-adapting strain of Infestation.
Besides, the Void and Infestation are very, very distinctly different. The Void is some unknowable, corrupting entity that hungers and longs to consume creation. It's magical, and a very dark form of magic, no less. The Infestation, on the other hand, is quite literally a bioweapon. A hive-mind, self-propogating strain of insatiable flesh and viruses.
And then, of course, there's another distinction: Kai'Sa is a void-huntress. A survivor. "The Girl who Came Back". Nidus is a rampant infestation. A tool of extermination. A living plague.
So...Tell me again, how Kai'Sa is a ripoff of Nidus?