Genetics - probability help?!?

If we're talking about twins here there are two options. If twins are identical the result would be the same because the fertilized egg and the sperm that fertilizes the egg would be the same cell. So the probability is surely 25%. If twins are non-identical the probability would still be the same but the same disease wouldn't necessarily occur at one twin if the other one has it too. So we would be talking about two different 25% probabilities. If they have a child after they had their first child the probability would still be 25% because events are completely independent. There might be some genes which differ from first fertilization to the second and so on. But I do not know any and did not hear of any.