Maybe she nees more litter box choices while the cats are indoors during the winter.
If it turns out to be totally behavioral due to the other cats, you can try the medication I am now using for 2 of my cats......Amitriptylin. (kitty prozac) I am having success with it and she would only need it during those months that the others are indoors. You'd have to go through your vet.
But maybe you can provide her with more "places" to get away from the other cats. i.e. cat trees, houses, condos.