She is clearly unwell and may benefit from being brought into the house for warmth and monitoring and some TLC.
Do you have a rooster? She looks like she has a bit of mating damage to her feathers.
It is concerning that her sister died recently and suggests there may be something more serious going on.
How does her body condition feel? Is she quite small and thin? The best place to assess this is the breast area. If her breast bone down the middle of her chest feels quite sharp under the skin, she is underweight and there is an issue with food intake or metabolism and will benefit from a heat source. If she is reasonable well covered and you can feel the breast bone but it is more rounded, then that is less concerning.
As azygous says, check the crop when she roosts at night and then first thing in the morning. If it hasn't gone down, it may hopefully be a crop issue, but it could also be a blockage lower down her digestive tract. The next place to check is her vent. Look for any swelling or pulsing or any abnormal bulges. If it appears to be pulsing, an internal exam to check for a stuck egg might be a good idea. This is done by carefully inserting a gloved finger an inch or two and feeling for an egg. It might be a normal egg or a soft shelled egg. The latter are particularly difficult to lay and take a lot out of a bird trying to push it out. and can make them feel very under the weather.
If you can come back to us with more info, we might be able to suggest what you can do to help her.
