Ask a Lawyer and Get Answers to Your Legal Questions
I am sorry to learn of your situation.
Unfortunately, I cannot help you find your brother's wife on this forum (we can only provide "general legal information" here), but I can help direct you to the proper individuals or professionals that can do so for you.
As you are attempting to locate an individual for a legitimate legal purpose (distribution of estate assets), you can utilize a process server or private investigator - both of which have access to proprietary databases (using government information - DMV, USPS, etc., and financial institution resources - credit reports, bank addresses, etc.) to help locate an individual for these purposes.
To find a reputable process server, contact your local (County) bar association and ask for referrals. Most charge a minimal fee for this (it is commonly called a "Skip Trace"), they will need some information about what the basis for the search is, etc. (part of their ability to keep access to these databases is ensuring that they have sufficient protections for the data that they are accessing - protecting individual information is part of their contract with the providers), but you can contact the process servers directly and see what they require for their business specifically.