Why two siblings that are not identical twins have the same alleles

your genes play a big role in making you who you are but brothers and sisters don't look exactly alike because everyone including parents actually have two copies of most of their genes and these copies can be different parents passed one of their two copies of each of their genes to their kids

another reason can be non-identical twins are result of mother releasing two eggs at once both of which become fertilized by different sperm resulting in two separate zygotes which develop into two children Like most full siblings non-identical twins share 50% of their DNA

Hope this helps