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 is His slave and Messenger.
The blood relationship between the seller and the buyer or between the intermediary and the buyer does not affect the lawfulness of the fee of the intermediary as long as there was an agreement and consent between the seller and the intermediary who acts as a mediator in the sale process in return for a known fee.
Therefore, if this percentage that you mentioned is taken from the price of sale that is known by both parties when conducting the brokerage contract, then there is no problem in this.
However, if the price was not known and it was agreed that the intermediary would get this percentage from the price for which the land would be sold, then the price was unknown, and this invalidates the contract according to the view of the majority of the scholars. Nonetheless, some scholars hold the view that it is permissible for the fee to be a percentage from the (unknown) sale price.
In any case, the mediator (broker) does not deserve his pay unless he did what was agreed upon in exerting his efforts to complete the sale process.
Allaah knows best.