The old history of the property doesn't matter. Your mother and her siblings inherited it recently. Their basis is the fair market value on the date of your grandmother's death. Whatever happened before that doesn't matter. The best way to determine the fair market value on the date of your grandmother's death is to get a professional appraisal.
The three siblings' gain on the sale will be what they sell it for, minus their basis, which is the fair market value on the date of your grandmother's death. They will each report their share of that gain as income on their tax return. In other words, they pay tax only on the increase in value since your grandmother died. Gain on the sale of inherited property is always considered long-term, no matter how long they actually owned it.
I don't know what you mean by the taxes "besides long-term capital gains." The tax on the long-term gain is the only tax they will pay.
