There are a few possible explanations for this scenario. One possibility is that the two girls are not biological siblings but were born to the same mother through different means. For example, one could be the mother's biological child, while the other could have been adopted or conceived through assisted reproductive technologies such as in vitro fertilization (IVF) or surrogacy.
Another possibility is that the two girls are identical twins, but one of them underwent a gender transition later in life. Identical twins have the same genetic makeup, so they would have been born on the same day, at the same time, in the same month and year. If one of the twins later identified as female and transitioned, they would still be siblings but not twins in terms of gender.
Lastly, it's also worth considering that the statement may be a riddle or a trick question, where the answer lies in a clever play on words or an unexpected interpretation of the information provided. In such cases, the explanation may rely on a creative twist rather than a realistic scenario.