Re: Imam
Throughout the Qur'an, the chosen families of the Prophets (a.s.) before the time of Muhammad Rasoolullah (a.s.) are mentioned and discussed. Allah has told us of the selecting of the personalities who are included within these noble families:
"Surely Allah chose Adam and Nuh and the descendants of Ibrahim and the descendants of Imran above the nations. Offspring, one of the other; and Allah is Hearing, Knowing." (3:33-34)
The implication is such that there is a chosen lineage - a pure bloodline in which its members display a high level of excellence. It is not correct to assume that the term "descendants" is to be taken generally. Allah is speaking about an explicit group of exalted personalities. In fact, He has chosen them, as is seen in light of the clause "Surely Allah chose...". Allah would not choose a wrongdoer amongst these families as being one of those who are "above the nations".
Allah further says: "And We gave to him Ishaq and Yaqoub; each did We guide, and Nuh did We guide before, and of his descendants, Dawood and Sulaiman and Ayub and Yusuf and Haroun; and thus do We reward those who do good (to others). And Zakariya and Yahya and Isa and Ilyas; every one was of the good; And Ismail and Al-Yasha and Yunus and Lut; and every one We made to excel (in) the worlds: And from among their fathers and their descendants and their brethren, and We chose them and guided them into the right way." (6:84-86)
It is a chosen lineage. The members who are within this lineage are chosen by Allah specifically and are guided by Him.
Allah also shows us in the Qur'an the roles that these family members played in the lives of their respective Prophets (a.s.). For example, the Qur'an discusses in many places the story of Nabi Musa (a.s.) and his brother Nabi Harun (a.s.). Or take, for example, the story of Nabi Ibrahim (a.s.) and his Ahlul'bayt (a.s.). There are many other examples in the Qur'an which relate the proximity and status of these chosen family members to the very Prophets (a.s.) themselves. The evidence is clear - it shows the importance of the elevated members of the families of these Prophets (a.s.) to their mission, which is to spread the the guidance of Allah.