Hi and welcome to the forum
Where are your girls from? It's natural for rats (especially kittens) to be nervous for a while when they go to a new home - all those different sights and smells coupled with a new home can be overwhelming for them. Rats from a good breeder usually come round quickly, but even pet shop rats that haven't really been handled much should come round with patience. If Brenda is more confident, then that in itelf can be useful for Lettie.
Although it may sound unkind, one of the best things you can do at the moment is take out all enclosed beds and hidey holes from your girls' cage, so that Lettie can't hide away. That way she will have to face her fears and she should gradually realise that even out in the open, nothing scary is going to happen to her.
Try sitting by the cage and offer something nice (such as yogurt) on a spoon, so she has to stay close to you to eat it and it isn't something she can run off with.
Another thing you can try is to put on a dressing gown or loose hoodie, and pop the girls inside whilst you sit on the sofa - try for at least 20 minutes a day. This will help them associate you with familiarity and safety.
I'm sure with time both girls will be bombing around like they've been with you forever
Pootle, Perkin, Bramble, Teasel, Marley, Oswald, Monty and Roger
Fizz, Cuddles, Bubbles, Horace, Ollie, Humphrey, Fergus, Max - "All the darkness in the world cannot extinguish the light of a single candle"