If you have branded Emily, you can trick her and take both (don't expect them to be happy, though).
If Emily likes you and you let her go away with her sister BUT you give her some food and money, she will come back in a few days.
After, like, ten more days (I don't remember exactly), Emily will ask you for help because Tisha is missing. Go rescue her and recruit her.
I haven't tested the other options (e.g. what happens if you let Emily go away without helping her and so on)