了 in this type of question is sometimes represented by other characters with similar pronunciations, such as 咯 (lo) or 啰 (lo). Such a question is used to make inferences about a situation; therefore, an optional 那/那么 (nà/nàme: ‘in that case’) is frequently used.
Lǐ：Nà gè nǚ hái gēn nǐ hěn xiàng，tā shì shuí ？
Li: That girl looks very much like you. Who is she?
Wáng：Wǒ gēn tā tóng xìng，kě shì tā bú shì wǒ jiě jiě 。
Wang: We have the same last name, but she is not my older sister.
Lǐ：Nà tā shì nǐ mèi mèi le/lo/lo ？
Li: Then (in that case) she must be your younger sister, isn’t she?
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.