Hi,
What the letter he asks is something illegal and has no legal holding as the child is minor now. Once she attain majority she has the right in the ancestral property , if any.
2. It is better to ignore him as there is no letter of assurance can be given other than an assurance by the mother that she will not ask anything for her daughter.However the mother can not sign anything what daughter has to do once she turns major , as mother's signing will have no legal value.
3. Ask him to prepare a letter and a legal document , tell him that your sister is ready to sign if he can get any document prepared.
4. There is no document can be obtained from court in nature of an court order stopping the child from inheriting from its father or ancestor's property.
5. The father can disperse his property as he wants while he is alive, nobody can get any share in his self earned property until his death ,so he does not have to worry about sharing it with his daughter. after the death of the father right will be equal for all legal heirs including this daughter..