The reason of why sexual reproduction has been maintained is a big question. About forty years ago Maynard Smith presented a concept of "two-fold cost of sex. Since then many hypotheses have been submitted. However the general answer has not been found yet. One of the hypotheses showed that the diversity of siblings was the cause (Lottery Hypothesis, Williams). Maynard Smith investigated it by computer simulation called "sib-competition model". He resulted that in the model the diversity of siblings was not able to overtake the two-fold cost of sex. Currently it is almost neglected. We revised the sib-competition model to shows the force that surpasses the two-fold cost. Further, we added the following three factors in the model to get closer to the reality: [1] milder selection, [2] environmental variance (VE), [3] reduced number of phenotypes. The result showed the advantage of sexual reproduction in some probable conditions. We think that the diversity of siblings may have the important roll for the big question.