Please don't let her out to see where she goes-now that you have decided she be an indoor cat only( btw- BRAVO) keep her safe and she'll live her full life expectancy,healthy & happy! It was not by "choice" that strays & ferals ended up living outdoors in the first place-its up to us to choose to protect and care for them in the best possible way( in a forever home)Luna is doing pretty well. She's eating well. She's definitely looking around the house for her sister, as she's usually quite timid, and she's taken to coming upstairs where the dog is to look around, trying to get in the garage, etc.
We've been having lots of cuddles and she's sleeping on top of my at nighttime. I think this has been helpful for both her and I.
Luna is unfortunately not leash-trained. She's used to being an indoor/outdoor cat, and since Starr went missing, Luna has become an indoor cat not by her choice. I have thought about getting her help to find Starr, and there was one night when I let her out where she ran to some houses, which I interpreted as places Starr might've visited. But I don't know if that's just my imagination.
I have heard of people playing sounds made by the "friends" of the missing cat. I might try that, or put Luna in a kennel to see if that attracts Starr. But I think I need to have a reasonable idea of where Starr is, as I would just be stressing Luna out for no reason if Starr is not in the area.
The issue is definitely having no idea where Starr is.