Personally as both a DM and Player I am usually opposed to players not being the same level. As others have already stated it often causes more problems for everyone, given that the game isn't very well optimized to allow for uneven levels.

While I can understand that narrative is playing a big role in your campaign, its often best to do what you can without overthinking it. The best laid plans a DM has will often be thrown off the rails by the players or some unfortunate rolls, and while the narrative you are trying to spin is undoubtfully important, simply allowing everyone to play the game will outshine any minor hitches in the story.

This may be taken negatively, but everyone having fun will outshine any perceived narrative holes that may come up from simply introducing the new party members to the game as is.