I was serving as executor for my late mother's estate in 2017. I made a personal payment to settle up back state taxes on the property.
For you to be able to deduct property taxes, you have had to be liable for them.
Back taxes were your late mother's so you could not deduct them.
If the Estate has other assets, you could get reimbursed from the Estate. Then the Estate could get a deduction for them (if an Estate return is being filed).
If they don;t and you inherit the property then the cost or basis in the property is the Fair Market Value at date of death plus the back taxes you paid to get the property current.
Sorry for your loss.