There is a great misconception in Zetsu's statement in chapter 606 saying that Nagato is a descendant of the Senjus because he is of Senju Lineage:
You must be registered for see images
It is not Zetsu's statement that is erroneous but the interpretation of others, if we take the translation of lineage based on the context of Zetsu's statement in conjunction with Kushina's statement in a previous chapter, the most accurate meaning of lineage, based on context, is the anthropological definition of the word: Kinship/Consanguinity:
You must be registered for see images
Kushina said the Uzumakis and Senjus were distant relatives, the true intended meaning of the word "lineage" based on the context of Zetsu's statement was : Consanguine of being related by blood, not Nagato descending from the Senjus : Nagato is a blood relative (lineage) of the Senju.
If the Senjus and Uzumakis descended from the same ancestor (Younger Son) then they will be close/immediate relatives, not distant relatives like Kushina said, which would contradict her statement and Manga canon, so it's logical to assume that the present day Uzumakis are "cousins" to both Senju and Uchiha since they did not descend from Rikudou himself but from Rikudou's relatives.
Also the original Japanese text of Zetsu in 606 is this:
You must be registered for see images
The original Japanese word that Zetsu used was "血縁" or Ketsuen which translates as "Blood Relative" NOT 子孫 or Shison which means descendant.
Anyone who can read Japanese will tell you that Zetsu's statement only meant blood relations but not pertaining to descent:
Originally Posted by Lady Mokusei View Post
Sorry Sasuke2, but you're wrong. Try to read original Japanese manga Naruto especially if your theory is based on one word. Here:
You must be registered for see links
Here we have chapter you mentioned (Chapter 606):
You must be registered for see links
Now carefully look at Zetsu's words. Ser Derp Obito is right about this chapter. Zetsu used word 血縁 written in hiragana like this けつえん. And this word has only two meanings:
1) blood relative or relationship
2) consanguinity