Siblings inherit 50 percent of their genes from mum and 50 per cent from their dad. Some siblings will inherit the same copies of genes while others will inherit different copies. Just like a pack of cards ,some siblings will pick out some or many of the same and others not or very little of the same. So those siblings who have inherited some same copies will look more alike and/or act more alike than siblings who have inherit different copies. It is very random.