All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The Prophet, sallallaahu ‘alayhi wa sallam, specified the Meeqats from where pilgrims are required to assume Ihraam. On the authority of Ibn ‘Abbaas the Prophet, sallallaahu ‘alayhi wa sallam, said: "…..and whoever is living inside these places can assume Ihraam from his own dwelling place, and the people of Makkah can assume Ihraam from Makkah." [Al-Bukhari and Muslim]
Based on this, you should know that your Meeqaat is Jeddah and your family's Meeqaat is Qarn Al-Manaazil or As-Sayl Al-Kabeer, which is the Meeqaat of the people of Najd. Consequently, it is permissible for you to assume Ihraam from Jeddah and then go to At-Taa’if and accompany your family, who will assume Ihraam either from At-Taa’if, which is before the Meeqaat, or from the Meeqaat which is Qarn Al-Manaazil and this is better.
It is also permissible for you to delay Ihraam until you come to At-Taa’if and there you can assume Ihraam along with your family or you do so from the Meeqaat. In such a case, you are of the same ruling of a person who passed the Meeqaat without assuming Ihraam intending to go back to a farther Meeqaat. This is permissible according to some scholars, as stated by the Shaafi‘i scholars of Fiqh.