Think of DNA as the blueprints for a house, down to flooring and paint color. Everyone is made from two sets of blueprints, for some features the blueprints compromise like blue paint and yellow paint becoming green paint for a room, and sometimes one set is dominate like tile floor instead of carpet. Children get a random set of blueprints from each parent.

So a mother could have green paint because of a copy of instructions for yellow and blue paint. But she passes along the instructions for yellow or blue paint, which then mixes with whatever the father contributes which might be white paint. So the children could have a dull yellow or light blue paint. With millions of pages of instructions it's unlikely two siblings would be identical but they will often share enough of the same traits to look similar.

And everyone once in a while something really weird happens and the instructions to build a window get spliced onto the instructions for a roof and then you get a skylight.