All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The predominant opinion of scholars is that anyone who builds on another person's land with his permission, or builds on a land that he shares with another person, is entitled to the value of his building only (i.e. not including the value of the land itself), for he has neither committed aggression nor usurpation. Consequently, none of these brothers and sisters, who built flats on their land, is allowed to drive out another from the block, especially that all of them did the same thing.
The solution to this problem is that you should either give your sister a satisfying sum in return for her share of the land, or to accept her as a partner in the block according to her share in the land and the value of the flat.
However, the eldest brother is right when he said that the elder sister was not entitled to build over the whole area. However, she, too, may say the same thing, for none of these partners has the right to build over the whole area of the land. None of them is even permitted to build over a part of it without the permission of his partners. This is not confined to the sister only, so none of them should have done so without the permission of his partners.
Reconciliation and reciprocal tolerance are the best solution for your problem. Otherwise, you should submit your case to Sharee‘ah courts of your country.
Allaah Knows best.