Emily can be found begging in the Red Lantern District as an unnamed Urchin Girl.
When you first see her give her some food and gold, then offer to bring her back to your mansion. When she takes a shower, wait for her to come out, don't rape her.
If you have prepared an aphrodisiac potion before offering to hire Emily, you will be able to spike her drink which will lead to a sex scene and unlock her sex menu. The Quest line will continue as normal from there.
Continue a few days praising her and being fair until a week later (e.g. day 8 if recruited at day 1) her sister will come to pick her up. Make Emily leave but secretly give her some supplies. She will come back a few days later.
If Emily's loyalty is in the "orange zone" by the time Tisha comes, you will first have to ask Emily to listen to her sister and leave, then give her supplies and gold.
After some time, Tisha will disappear. If you haven't unlocked Emily's sex menu, do it now by accepting to help in exchange for more intimate services. The quest goes on the same as if you had simply accepted.
Look for clues at the Mages' Order, then beat up the slavers to learn her whereabouts. You will need 500 gold for the next part. In Gorn, you will find Tisha being sold as a slave. Choose to buy her whenever you want, recruit her but do not brand her (the game even tells you), she will ask what reward you want, ask her for sex.
Asking for nothing plays exactly the same with a short paragraph added at the beginning.
You will now definitely have sex unlocked with both Emily and her sister Tisha. Neither will be rebellious. Emily will be extremely loyal.