The cere is a great indicator of gender with most budgies/parakeets. However, it is not reliable with many of the pastel or solid color mutations such as your white bird. In those birds, you have to try to determine gender by behavior. When you have multiple birds together, this can get tricky because same sex birds will often take on a pair bond and behave and even mate like a true male/female pair. There is actually a test called DNA sexing that is completely non-invasive and requires a few feathers or drops of blood if you are ever curious to know for sure. Since the previous owners were told Lana had laid an egg, it was probably true. Unless they bought it for a lot of money as a breeder, there would be no reason for someone to lie about a bird’s gender.
I would be careful about having three birds if they all share one cage. The brother and sister should absolutely not be allowed to breed – related birds can produce offspring with defects and American parakeets are badly inbred to begin with. Also, when you have an odd bird out, generally two birds end up bonding and will bully the odd bird. While yours are getting along now, I’m not sure if it is because they aren’t sharing a cage or if you have been lucky so far. I personally would split the brother and sister up or they will eventually begin to mate and that would leave you to have to destroy any eggs that resulted. They can all interact together out of the cage, but I wouldn’t keep these 3 birds in the same cage.
Thank you for asking Lafeber,