Father's loan repayment

One of the four sons gave money to his father.Father expired telling his elder son to settle all the dues of his son who gave money. Father appointed his elder son as a nominee for his shares and bank FDs. But elder son as nominee is partitioning shares and FDs among four sons and a daughter. Elder son says that they will give back only principal amount and even for that they all ask for a written statement that the loaned son would cooperate in all ways for taking heir certificate and selling properties in future .But the loaned son wants the loaned amount with 18 percent monthly compound interest from the father's big estate. They are not willing. Hence the son who loaned father did not cooperate for taking heir certificate.Legally ,how the Court looks into this matter.Whether 18% monthly compound interest will be granted by court.How long the case will take if one files a case? Whether brothers can refuse to give interest?Whether legally can the brothers ask the loaned son to give an undertaking for cooperation for all future procedures for the property? Whether without heir certificate and partition can they sell their share in the property and can they cause more trouble to me. Father's brother is still alive and he has 3 sons and one daughter and my expired father has 4 sons and one sister. Compromise through relatives failed. All properties are in the names of my father and his brother who inherited the property from my grandfather who got all properties by a will from my great grandfather.