Second, you must take Sara with you on this sidequest. Also, prepare for some trouble at the end since there's a boss fight and some annoying enemies, like Honey Pots.
To recruit Lucia
1) Have Sara in your party
2) Go to the mansion north of the entrance and watch the cutscene with Lily.
4) Chose the second of the three options
5) Talk to her again and choose the second option
6) Chase her around her mansion and when you find her, expect a boss battle.
7) After beating her up, go to the house across a wood bridge and talk to Lucia, she should join your party.
To recruit Lily
1) Climb the two stair cases to the right of the Inn and then cross the wood bridge (you can see Amira under it). Enter the house there and then procceed to the stairs.
2) Talk to Lucia and choose the second option of her dialogue, she'll vanish and now you must go after her outside of town.
3) Exit the village and go south to the tower, make sure to have a party member with Lockpick II because there is some green and blue chests around the place.
- 3.1) If you talk to the Oni Girl to the left she'll give you some tempura.
4) At the top floor Lucia will be waiting for you, when the choices appear choose the first one to begin the battle.
5) Go back to the village and enter the mansion to speak with Lily, she'll be at the table to the left of entrance.
You can still visit the other location for treasures and recruitment, since the monsters on the two places are different.
Also, you can change sides with Lily or Lucia until you win the boss battle against one of them. Just go talk to the other after choosing a side, that'll make the other go away to her place as a boss. I sided with Lucia first in this guide and then, to make Lily's quest, I warped back from her boss room and talked to Lucia, making her go away to the tower just to finish her off instead of her sister.
If you want to recruit both Lily and Lucia, you will need to do a New Game Plus, siding with the person that you didn't side with in your original playthrough. Note that for certain decisions later down the line (like the Succubus Village quest) the game only considers the person that you sided with in your curent playthrough.