brothers not complying conditions in settlement deed remedie

Hello sirs i am advocate sudhir , happy meeting you in LCI. Sir, my query is regarding a civil dispute. A person has 3 grandchildren I.e.2grandsond and one grand daughter. He had settled his self acquired property in favour of his two grandsons with an obligation on the grandsons to perform the marriage of thier sister from the income of the house property . Subsequently the settlor passed away. The two grandsons did did not perform the marriage as envisaged under the settlement deed. The granddaughter had performed her marriage from the income which she earned by running a tailor unit from the age of 14 years. settlement deed was executed in the year 1997. Now in 2014 there is a dispute within the family. I issued to legal notice for partition. It was replied by them that the settlement deed executed by their grandfather is standing in their favour and the granddaughter I.e. their sister has no share in the property. Only after the reply notice dated 10.05.2014 we came to know about the alleged settlement deed before that we were not aware of the settlement deed. Only than we received a copy of the deed from the SRO and learned that as per deed there is an obligation on the grandsons to perform the marriage of thier sister which was not complied by them. Can we challenge the settlement deed. If not what is the other remedy available for the poor woman who was deceived by her brothers.