one related by descent in a diverging line from a known common ancestor, as from one's grandparent or from one's father's or mother's sister or brother.
COMMON ANCESTOR: Kaguya
Diverging Lines: Hamaru and Hagoromo
Every child from Hamaru and Hagoromo . . . will be cousins, sisters or brothers.
Whether they be 1st, 2nd or third cousins. . . . . etc
I don't understand how . . . you think the children of two brothers cannot be cousins
I find that utterly fascinating.
No sadly ...you are still incorrect.
I'll put examples in the real world to make you understand clearly.
Kaguya - Adam & Eve
They are our Common ancestors because we all supposedly came from them. Even if you don't believe religiously you have to imagine anyway.
Son/Daughter of Kaguya aka Adam & Eve: Let's say Bob and Margert.
Every child from Bob and Margaret .... will be brothers, sisters, and cousins. Yes you are correct on that point, but this will only last for a while.
Sadly you are missing one thing. Scientists have shown that each human can be traced down into one single female DNA component, but that doesn't mean you are directly related to that person or have their traits. As this theory is shown DNA changes and mutates, and each person born is unique.
The problem with your statement is that Ashura and Indra are the true grandchildren of Kaguya, and their children for a while. However the genetic traits of Kaguya will slowly disappear.
Your DNA contains a record of your ancestors, but you aren't a carbon copy of any one of them. The particular mix of DNA you inherit is unique to you. You receive 50% of your DNA from each of your parents, who received 50% of theirs from each of their parents, and so on. If you go back far enough, there is a chance that you inherited no DNA from your past ancestors besides the parents.
With that being said. Ashura and Indra line of Uchiha's and Senju's are directly related to Indra and Ashura, but even though most Uchiha's and Senju's have no direct abilities or traits similar to them (Things like Kekkei Genkai, hair, eyes, elements,etc., are kept. I'll explain why.) are obviously passed down from each Uchiha before hand. Meaning Indra started the Sharingan gene, had intercourse, and then the child carried the next Sharingan gene, and then kept passing it down until the Clan bloomed (Senju aswell).
Uzumaki like we've said broke off from the direct Senju line which is FAR beyond Kaguya, by years and years. Meaning they have an even smaller percent ...well a 0% possibility of being related to Kaguya. They are related to Senju but if they survived and kept striving for another hundreds of years, the Uzumaki would have been what the Senju are to Ashura. Related and started from him, but not really him at all besides the obvious "Strong body" Kekkei Genkai traits that are passed down in NARUTO'S world. Not our world.
I'll be glad to answer more of your questions as you are particularly confused on the subject. If you still don't understand it, I'll have to break it down step by step, but I'm really happy to do so.