Even though the estate consisted of bank accounts, there still has to be final tax returns filed and tax approval which may not come until 9 months after death. Most estates can be settled in one year, possibly 2 for more complicated estates.
Any distribution can be made now, but it will be at risk if final tax approval has not been given. That means that if the tax authorities say more money is owed and there is not enough in the estate to cover it, then the executor would have to ask the beneficiaries to return the money or property. If not the executor can be personally liable and that is what I mean by at risk.
If your daughter-in-law is truly concerned, then she needs to consult with a probate attorney in the county/state where her mother lived just prior to death. I am not licensed in New Jersey and am not familiar with their procedures so your daughter-in-law needs to confirm what i have stated.
However, your son is not an heir and its not any of your business. Your son's wife will get the money evenutally and if she wants to keep it separate its her property, not your son's. If your daughter-in-law does not seem concerned then its not your place to intervene.