Are Cloud and Kadaj and Loz and Yazoo and Sephiroth brothers or family?

Answer 1

Yes and no. The way I understand it is that Cloud, Sephiroth, and the three Remnants are only really related through sharing Jenova cells. Sephiroth was injected with Jenova cells when he was in the womb and upon discovering as an adult, the false truth about Jenova being his mother, went insane. After displaying an unprecedented amount of strength and defeating Sephiroth, Cloud was experimented on by Professor Hojo, who infused him with Mako and Jenova cells.

While in the Lifestream, Sephiroth tried to resurrect himself, but because the memory of his physical form was somewhat lost to him, he was unable to give himself a new body. So he conjured the forms of three young boys, Kadaj, Yazoo, and Loz, into whom he poured all of his pain, anger, and hate. Collectively, the boys care called the Remnants, and though appear to have their own thoughts and feelings, act on Sephiroth's will. He unleashes them out into the world to find Jenova's remains so Sephiroth can ultimately use Kadaj's body as a vessel to return to life.

Kadaj, Loz, and Yazoo call themselves brothers, and are fairly correct to do so. And considering their creation, one could call them Sephiroth clones - as they are referred to throughout the FFVII anthology - or one could say that Sephiroth is somewhat like a father to them in that he created them from himself. They call Cloud their brother, presumeably because he shares Jenova cells, though the truth is poorly explained. Furthermore, the Remnants refer to Jenova as "Mother" simply because they're acting on Sephiroth's will and since together, they are in deed Sephiroth himself, they also have fragments of his memories and are filled with his own emotions; so in the end, since Sephiroth thinks Jenova is his biological mother, the Remnants do too.

This is all I have for now. I hope this was helpful to you.