When two people, who were raised in the same household, are faced with the same decision, what major factor can explain their different decisions in life?

No two people are exactly the same including even twins as they both have at least slightly different experiences of life at first in the womb by their particular position in it and then once born and living in the same household, their interactions with the different members of the household will vary depending on their age, sex, what part of the house they are in at any one time etc thus their decisions will be quite individualized according to their being and their needs.