There was a book I read in the 1970's about "sociobiology" that explained this. If I remember right: People share exactly 50% of their genes with their parents and children, but only probably 50% of genes with siblings -- it could be 51%, or could be 48%. To preserve their own genes, people will feel altruistic to give their lives for three of their siblings (about 150% of their own genes saved), but not for one sibl…
I don't see why the percent shared could not be higher than 50% with your parents or children. You can share a gene with someone that you did not get it from or give it to. For example, suppose some particular gene has three variants, V1, V2, and V3. If your mother has V1/V2, and your father has V2/V3, the possible cases for you are V1/V2, V1/V3, V2/V2, and V2/V3.
In two of those cases you share 50% of that gene with each parent. In the other two cases you share 50% with one parent and 100% with the other parent.