So, OP the facts are:
You have a half brother who you love very much.
You both share a father, who is still alive. Both of you brought up by your mum and dad. But your DB's relationship with your mum was not good and they haven't spoken for last 20 years.
When your DB's mum died he received an inheritance from her which he didn"t share with you. But you didn't expect to share so no problem.
Your mum has recently died, but did not have anything to leave anyone.
Your grandmother then died soon after, and you have inherited a significant amount from her.
Your DB is now saying that you should give him half because if it had been left to your mum, he would have received half of it. Because your mum told him so before she stopped talking to him for 20 years?
You can see that your brother is lying, right? Plus whatever your brother is saying it is also irrelevant, as you did not inherit from your mum, you inherited from your grandmother.
You can love your DB, but it does not stop him from being a scheming, despicable brother.
Being as nice as you sound, you would have given your brother something, unasked, despite his actions with his inheritance from his own mother.
However, given the immense pressure he is exerting on you, you need to take a step back and look at your brother and realise he is no DB. He is using spurious, unfounded reasons to try and grab half your inheritance. For that reason alone, do not give him 25% at all. If he threatens to abandon your relationship, tell him bye, bye then. Believe me, he is showing you his true face, so take note and ask yourself, would you tolerate such behaviour from your best friend? If not don't tolerate it from him.