With the arrival of Ethan, your nephew Aidan will probably start to get jealous (more or less) of his baby brother. I studied Child Psychology for a little bit anyways although I don't think I have enough knowledge to really give any professional advice but I do remember some of the phases of Child Development. Ausubel's theories on Satellization, Desatellization and Resatellization (although on the net there's some difference to the defnition of the phases I learnt in a textbook). There is two more step before Satellization which is Self-Realization and Pre-Satellization.
The phases:
Self-Realization: Basically is the recognition of self. Acknowledging your own existence and recognising yourself in the mirror.
Pre-Satellization: This is where the toddler thinks that world/universe revolves around him/herself. They know they need only cry and they will get parents running to them. They leave this stage when parents enforce rules. Such as saying "no" to the child's demands.
Satellization: The toddler reaches this phase when they revolve themselves (their world) around the parents. They start to do things to impress the parents. They seek recognition and 'pats on the back'.
Desatellization: Adolescence kicks in. They no longer neccessarily think that their world has to revolve around their parents. They have other 'centres' to revolve around: friends or teachers.
Resatellization: Young adults reach this phase when they start having families. Some of us yearn for this but I think Resatellization is being postponed due to career advances these days.
Your nephew Aidan is entering Satellization from Pre-Satellization or has just entered Satellization. If I remember correctly if a new Sibling arrives around this time, the child entering Satellization may not enter it properly and will take some of the Pre-Satellization characteristics with him/her and become a difficult child. He/she will misbehave to seek attention from his parents. There's a simple solution to this and that is get the child involved with helping out with the new baby. By helping out with the new baby, the child will 'satellize' around not only the parents but the new sibling as well. The child would fulfil his/her needs at this phase and will not need to seek attention by misbehaving. Actually this Satellization phase is quite important, if not entered correctly the child can be quite difficult to manage. Spoilt children are believed to not enter this phase at all. They remain in the Pre-Satellization phase and from some of the people I've met, I think some remain in this phase all of their lives hehehe...
Of course this is only a theory and very general...and human behavioural development probably could not be summarised in just these phases...
Anyways I'm not a Child Psychologist so I should really shut up...I can't believe I can still remember this kinda stuff... maybe I should've studied Psychology... but then I don't really like collating and processing statistics...