When kaname had again turned yuki into a vampire then is about to kill kaname then yuki told that he is a elder brother of her then how can be she be his phiyonse to get married

In the story, it's traditional for pureblood siblings to marry each other, as they believe this keeps their families pure. Haruka and Juuri (Yuuki's parents) were also siblings. The incest isn't taboo for vampires since their healing would prevent any children being born with genetic diseases.

Also, the vampires consider themselves beasts and "that's what beasts do". That was how Kaname explained it when Yuuki initially freaked out over being in love with her brother.

Thirdly, Kaname later reveals that he isn't really Yuuki's brother. He was the family ancestor, who was awoken by Rido using his real nephew's blood, and Kaname was so starved of blood at the time that he had to revert to an infant to prevent himself from attacking the family. Juuri and Haruka then raised him as their own son. Yuuki believed he was her actual brother until he told her the truth. For some fans, this distance in their genetic relation stops them from seeing the relationship as incestuous.