Armed burglary is a first degree felony punishable by up to life in prison. Grand theft can be a 1st, 2nd or 3rd degree felony depending upon the amount and the items that were stolen. Dealing in stolen property is a 2nd degree felony punishable by up to 15 years in prison. Additionally, if your nephew was in actual possession of a firearm at the time of the burglary there is a minimum mandatory sentence of 10 years in prison.
The fact that he has no prior record will be helpful, but he is going to need an attorney to review the case and determine what if any defenses or legal issues there may be in order to get the best result.
Ms. Jacobs is correct in her assessment of the potential range of penalties. Depending on the age of your nephew, there may be ways for this case to be resolved that result in a reduction of charges and the possibility of youthful offender sanctions. If that were to happen then the cap on incarceration would be six years. However, as presently charged I do not believe youthful offender sanctions would apply. Additionally, at the time of sentencing, the defendant must be younger than 21.
On a separate note, I know the situation may look grim, but it is definitely worth considering hiring an attorney to review the case and to make sure the evidence is present to secure a conviction prior to simply resolving the case.
Both of the above answers are correct and collectively comprehensive. The only thing I would add/ask is what he (or his possible codefendant) was armed with. If the answer is a firearm, then there could be 10/20/LIFE implications. If the answer is something different, then at least there are no minimum mandatory sentences to worry about.