My father died in 1964, with a will; leaving an undivided one-half to my mother and an undivided one-half to me and my two sisters.
She remarried about a year later, and she and her new husband somehow either sold or somehow got rid of the property.
I don't know how (maybe you can answer), how anyone who thinks they own that property can have a clear title.
I was contacted years ago regarding this matter, but was told we (sisters -heirs) would have to prosecute our mother. She passed away last year and I want to know the best approach to take regarding recovering what is legally ours.

Thank you for any advice/assistance.

I'm not sure anything can be done at this late date. Yes, your mom and her new husband committed a fraud by selling the property without the permission of you and your sisters. But whether that's enough to overturn the sale 50 years later is a large question.

The bottom line is you need to go to court to attempt to overturn the sale. If the purchasers or current owners bought title insurance, you may be able to get a settlement from them to recover something. So I suggest you consult an attorney in the parish to see what can be done.