1) For a partition suit involving immovable property, the valuation must ideally reflect the prevailing market price or the government circle rate (ready reckoner rate) at the time the suit was instituted.

2)  plaintiff initially has the freedom to estimate the value of the relief, the court has explicit powers to investigate if the property is demonstratively undervalued or arbitrarily valued to evade court fees or manipulate jurisdiction.

3)  You have the right to file an objection (by filing an application under Order VII Rule 11 stating that the property is grossly undervalued. The court will then order an inquiry, look at the circle rates/market value, and determine the exact correct valuation

4) your sister in law has to file amendment application under order Vi rule 17 explaining why the valuation needs to be changed 

Whatever value of the suit property mentioned at the time of of instituting the suit will continue till it is disposed unless the defendant challenges the same during trial seeking dismissal for undervaluing the property and paid less court fee.

But the responsibility to prove the higher value of the suit property with the support of substantial documentary evidences will lie on the person disputing it.

Yes she can change and pay the deficit court fees. You can also point out the same. The court will value the property and later ask her to pay the same. The valuation depends on the ready reckoner rate and not the market rate 

Under Section 7(4)(b) read with Art. 17, Sch. II, a plaintiff in suit for partition  of ancestral property is required to pay fixed Court fee and not ad volrum Court fee, Market value of property in a suit for partition  is not relevant.