Succession Act Rules on Intestacy: [broken link removed]
Your Dad's Estate.
Therefore when your Da died, your Mum was entitled to 2/3 of the estate with the balance divided in equal shares between the kids (1/3 between the four children(1/12 each) - as your brother was still alive at this point).
Your Mum's Estate(includes the 2/3 portion of your father's estate that she inherited)
Easy-peasy: divided equally between four of you.
Your brother's Estate:
He was entitled to 1/12 of your Father's estate and is entitled to 1/4 of your mother's estate as he died after them both. The destination of his estate depends on whether or not he made a will. If he did, then the Will decides, if not, the Rules of Intestacy apply and his wife will get 2/3 with the rest between the kids in equal shares