My wife's sister claims to be the executor named in her aunt's will and that she is the sole inheritor. Her aunt's estate is not planned to go through probate so the will has not been filed with the county court.
There is a process available in the Texas Probate Code to compel someone in possession of a Will to deposit it with the county clerk for safekeeping. Once that occurs anyone can get a copy of the Will. Then you can make a decision about whether or not it should be probated. You will need a lawyer. You cannot do this pro se.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
You must be a distributee under the will or an heir at law, assuming no will exists, to have an interest to justify forcing the production of the will. I don't know if your wife is an heir in the circumstances you describe without more information. If no probate is planned, that suggests there is no probate estate and the aunt is receiving the assets by beneficiary designations or right of survivorship. If no probate is necessary, the terms of the will then are moot.
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You should hire an attorney to help with this.
This answer DOES NOT establish an attorney-client relationship. Please contact me for help and a more complete answer. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.