1. Even though it was your grandfather's self acquired property after inheritance of his share your father becomes an absolute owner of the property he inherited.
In that case, he can decide about distribution of his properties in the manner he may desire and decide and transfer the properties to who ever he may choose, including any member outside his family.
There is no obligation on your father to be under pressure created by your brother.
The logic of your brother is not justified.
It appears he is not willing for an amicable settlement,but if he chooses the legal route then he will be loser because his claim is not legally maintainable or tenable in law, as neither he nor you have any rights in yor father's property at lest not during his lifetime.
2. It is absolutely an unjustified demand.
If he has availed the loan, then it is his responsibility to repay the loan.
He cannot legally insist on you to repay the loan in any percentage.