For example - Let's say a mother has a billion DNA packets she could pass along. A Dad has a billion DNA packets. Packets can be from long ago people - passed on from a million past ancestors. When they conceive - the baby will have a billion packets too. Which that leaves 500 million unused packets from both parents. Kid two picks up a billion packets - but it's not possible they inherited the exact billion packets as the sibling 1. You could literally have 1 trillion children from the same two parents - and none of them would be the exact as another child.
Sequence of the packets would further complicate the math.