When your father made his last will did he employ a solicitor to prepare it ? And who witnessed it ? Were the witnesses wholly independent e.g the solicitor's clerks ?
A solicitor is bound to satisfy himself that the person making the will (testator) knows what he is doing and acts freely in making it. He would normally enquire what children the testator has. That's because he is alive to the risk of claims being made, not simply because a child is left out but because a child, mentioned or not, might have some claim, as, for example, a dependant, to something more from the estate [some people have such a claim if they were being maintained by the deceased].It is, in any case, rather unusual for someone to keep a child out of their will. He would be at pains to prevent future litigation, however valid or however futile, and would probably ask why the instruction to omit one child in favour of the other was given . That being so, a statement from the solicitor is valuable and may be fatal to the claimant's case.The solicitor is bound, on request, to provide you with a statement of the circumstances of the will being made.
Duress, that the testator was forced against his wishes to make the will a certain way, is almost impossible to establish. The claimant is, therefore, reduced to saying that the testator didn't appreciate the nature or extent of the gifts he was making (i.e he misunderstood the words in the will or the overall effect of them). That seems highly improbable in a will as simple as this must have been, effectively 'I give everything I own at my death to my daughter'! Anyway Judges aren't concerned with stopping human perversity in wills; they may find behaviour perverse or eccentric, not something they'd do, but if it is done freely and knowingly it stays done ( that's a general principle, not a comment on your case !)
I suspect your brother is trying it on in the hope that you'll give him something to go away.