It is often said that ‘between’ is used for two items and ‘among’ is used for three or more. However, this is not completely accurate. ‘Between’ is used when naming distinct individual items which can be two, three or more. ‘Among’ is used when the items (must be three or more) are part of a group, or are not specifically named. In the given sentence, more than two distinct individual items are being talked about i.e. daughters and sons. Thus, ‘between’ will fill the blank.