Dear Liza: My brother died a few days ago and hadn't updated his will since the 80's. His wife has Alzheimer's. They named each other as executrix, however they did durable POA on each other naming my sister-in law's cousin. Do we need to go to court to be able for her(representing my sister in law) to access the deceased accounts that are in his name only? Like so many of the excellent questions that I get on this blog, the answer is, I'm afraid, "It depends." Your sister-in-law is the executor of your brother's Will. Your sister-in-law is incapacitated by Alzheimer's. So, you are correct in looking to her Durable Power of Attorney to see who is legally authorized to act on her behalf. I always tell my clients that the Durable Power of Attorney is the "not dead yet" document--for those who grew up, like I did, watching Monty Python, you'll get the reference. My point is that a POA is to be used when the Principal, the person who signed it, is not dead, but unable for some reason to manage their own affairs. You do not need to go to court for the Agent (your sister-in-law's cousin) to have authority to act for your sister-in-law with respect to her assets, provided the POA is valid and authorizes her to do so.
But, here's where it gets a bit more complicated: the assets in your brother's name may need to go through probate before they can be transferred, or used for, his wife's benefit. He's dead, so his POA no longer is the controlling document (the Will is). What you need is someone legally authorized to act on behalf of your brother's estate--and that depends on whether or not you need a probate. If you do, the court has to appoint that person. If you don't, your sister-in-law's cousin may be able to act on your sister-in-law's behalf under that POA, but only if the POA authorizes such action.
The necessity of a probate depends on how much he had, where you live, and what his Will says. You need to find out what the 'small estates' procedure is for your state--in my state (California) assets worth less than $100K in total (with an exception for real property which I won't get into) can be transferred without a probate procedure, but that limit is different in each state. It also matters what your brother's Will says--did he leave everything to his wife? Did he give it to her outright or in a trust? This will also determine whether or not a probate is required and what kind of probate (many states have an expedited procedure for surviving spouses). If a probate is required, your sister-in-law's cousin could petition to be named as the executor, since your brother's wife isn't capable. But again, it depends whether he named a second choice in his Will, and whether or not that person wants to serve.
If it turns out that your brother's estate doesn't require a probate, your sister-in-law's cousin, acting on behalf of your sister-in-law as her Agent under the POA, may be able to transfer the assets, but you'll need to use your state's small estates procedure to do that and the POA would have to permit such action.
It sounds like consulting with an attorney would be worth it, since you've got several issues to unravel. Good luck.