Your mother is entitled to her legitimate rights out of her father's property for her 1/6th rightful share in it.
Since for filing any case appropriate court fee have to paid you cannot claim any exemption towards it.
The court fee is a local subject, since you are already into the litigation you may enquire about the same locally.
The court fee is not refundable until the suit is withdrawn as not pressed.
The court fee shall be paid for her 1/6th share only that too as per government guidelines value only.
If you are unable to pay court fee then may get an exemption through a decree passed in the pauper petition case that can be filed by you in this regard.