Similar thing has happened in my family on my fathers side. His mother and her sisters all look different - not necessarily a giveaway.
However, looking at her fathers military records, there are quite large discrepancies in conception/birth dates (unless there were particularly long and short gestation periods!).
Unfortunately, people will seek the affections of others, and unless as stated the surname of the biological father is particularly unusual, then it's likely to be a dead end. We never found out who the man involved in our family was, but we're 100% certain it's not the same man for all five sisters
