Patience, patience, patience. Just continue what you're doing and let her come to you. Even cats who are not feral, can be very independent. I had a cat "Jetta" was very independent. You couldn't just go pick her up. She was that way for the 4 years I had her. She got outside twice and had to litters. We eventually gave her to a lady who had a huge house and lost her own cat. She took Jetta and Lea (who we thought was Leo at first). She just let them be themselves. As long as Mischief is using the litter box, that would be my only concern. She should eventually join in play with you and her sister.