All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
This girl may be your sister and she may not be your sister. Claiming family lineage is not an easy matter because its confirmation leads to many rights, like keeping ties with kinship, the right in the inheritance and other legal rights. Therefore, you may confirm this matter from your father whether or not this girl is your sister.
However, if it is confirmed to you that she is a legitimate daughter of your father, then she is your sister, in which case you should keep ties with her and endeavour to rectify her if she needs this.
Allaah Knows best.