Characteristics such as flower colour or blood group are controlled by genes. Different versions of the same gene are called alleles. For example, the gene for flower colour in pea plants has one allele for purple flowers and another allele for white flowers. Different alleles code for slightly different proteins, and this can produce different characteristics.
Each gamete carries one allele of each gene. An offspring receives one allele from each parent, so it inherits a mixture from both parents. A dominant allele affects the characteristic even if only one copy is present. A recessive allele only affects the characteristic when two copies are present, one from each parent.
Brothers and sisters from the same parents can look different. Meiosis gives each gamete a different mixture of alleles, and fertilisation is random, because any sperm could fuse with any egg. So each child inherits a different combination of alleles, and this is why inherited characteristics differ.