Son does not want property left in will

Hi All,

My father died 12 months ago and left his 50% of a property in Spain jointly between my mother and 5 adult children. He had a Spanish will as does my mother. one of my brothers does not want his 1/6 share and refused to sign the power of attorney re this. All other paperwork has been completed as per Spanish requirements.

My mother is left that she can't finalise the paperwork and is going to have to pay penalties until the situation is resolved. Any ideas what can be done to make him sign something one way or another or is there any way around this.

Thanks a newbie here !!