Is it permissible for those coming from Riyadh to perform Hajj from Jeddah if their intention is to first visit their brother in Jeddah, and how should they enter into a state of Ihram in this case?

It is permissible for someone intending to perform Hajj and wishing to pass through Jeddah first to go there without ihram (definition). Then, they should return to the miqat (definition) of Qarn al-Manazil (Wadi as-Sayl) and assume ihram from there, after which they can join their campaign in Mina. However, if the journey is by land or air, ihram must be assumed from the miqat before reaching Jeddah, based on the Prophet's (peace be upon him) saying: "These miqats are for them [the people of these places] and for those who come upon them from other than them, among those who intend Hajj and Umrah (definition)." There is no harm in passing the miqat with the intention of returning to assume ihram from it, as ruled by Sheikh Ibn Uthaymeen (may Allah have mercy on him).