To me the problem is of a different nature. To maximize the greatest good for the greatest number, neither you nor your sibling (or total stranger) should press the button.
As was stated in the original dilemma, the wizard who is holding you is evil. Assuming you are good, or more importantly in Baldur's Gate (I or II), if you are lawful good you cannot press the button. By your devotion to law and the good, you realize that the very proposition that you would participate in this evil act would trigger your higher brain functions to think about the proper course you should choose. Your first thought may be that sacrificeing yourself would be the greatest good, but it is not. If you and your sibling both refuse to press the button, neither of you has done anything wrong. In fact, you've kept both of you alive for the moment. The responsibility for your act lies with the evil wizard, and not with you or your sibling. You are simply lawful good, you are not neccessarily a hero. And what would you gain by being a hero, other than death? You would gain a lifetime of guilt for your sibling, and she might go on to exact revenge (which is not considered good in our world, and its inherent goodness in D&D is open for discussion). The point is, if you are both concerned with the greatest good, you would not submit to an evil act, and would remain alive until the end of the hour, when the wizard returns to kill you both. But his act is his evil, whereas the evilness of suicide is debatable.
So if you are good, you cannot press the button. You must allow the wizard to be evil, while you and your sibling uphold the good.