NTA, but you're going to have to be careful from here on out. If people think you're abusing your royal power, another noble who is an asshole might see that as justification for a revolution.

First, DO NOT wear the crown or call yourself king. Instead, use a title like 'Steward' or 'Regent'. This will assure people that your reign is temporary and that you're not just grasping for power. You should also announce a date for your nephew's own coronation in 10-15 years, for similar reasons.

Second, avoid making radical changes to the kingdom. It ultimately belongs to your nephew, and you should not tamper with it beyond necessity. Any part of your personal agenda, or the fact that you have an agenda at all, may be used against you. "If it ain't broke, don't fix it" as the peasants say.

Third, be involved with your nephew. Spend time teaching and training him, but also play with him. Allow, perhaps even encourage a hobby of his choice beyond his official duties. You want to cultivate a relationship of mutual respect and trust. This is true for any parent, but especially when your future may be directly in their hands. Honestly it sounds like your brother and sister-in-law got you off to a good start here, don't mess him up.

Maintain these for the 10-15 years and you should be able to abdicate peacefully. From there, you should be in a great position for an advisor, with years of experience in ruling as well as the inherent trustworthiness of a family member. You'll have many of the benefits of being royalty while no longer having the responsibilities that come with the crown.