Two things come to mind. Were all the children adults? I’d hope so because minor children couldn’t give proper consent to surrender their legal rights to an arbitration court based on sexist principles that are otherwise illegal in the UK.

Second, why on earth would daughters agree to this unless pressured, since the arbitration under Sharia law guaranteed that that would not receive equal distribution of property. I suppose they could feel culturally or religiously inclined to follow Sharia law, but I do have a problem with binding arbitration which uses a basis that would be impermissible in a court of law to decide the outcome of cases.