Secrets like this can be really crazy, something similar happened to a friend of mine's family.
A Lady showed up with a son that was supposedly my friends father's child and before we knew it his father went on to marry the lady and they built a house very close to my friends house, its really crazy but I am happy yours did not amount to that and your sister's existence did not cause a friction in your family.
Now that's crazy! I think my mother would have gone crazy if she found out that the woman my father left with lived near our house. Thank goodness my family took things more relaxed, or at least that's what I perceived. Anyway, everything went very well.