When your wife's aunt made her JTWROS, they each owned half the property (with each half being worth half what aunt paid for it).
When aunt died, your wife owned the entire property, one half at original cost basis when gifted, the other half at the 'stepped up cost basis' amount.
Add the original cost basis of the gifted half to the stepped-up basis of the other half for Cost Basis at time of sale.
Your wife's gain on the sale would be Net Sale Proceeds - Gift Cost Basis (1/2) - DOD FMV (1/2).
Click this link for more info on Cost Basis of JTWROS Property.
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