Hi Darel,
I do not have much experience, nor any education about HD, to offer advice to your situation, but I will tell you that you have come to the right place to seek guidance if that's what you're looking for!
Perhaps to get some better advice from people in this community, let us know more about your sister and her fatigue.
Is she seeing a doctor for her HD specifically?
How long has she been feeling this exhaustion? When you say "it's been a couple of years", do you mean since she got tested?
How old is she?
What is her current, even past, lifestyle like?
For my boyfriend, he has the same feelings of exhaustion, and I'll tell you, it can be catchy! So, hopefully it is not getting to you too much.
The only thing I can really tell you that I see helps improve feelings of fatigue are eating right and keeping active.
When he eats (what I call) "real" food, he looks and acts healthy. When fake or processed foods are consumed, fatigue gets worse. Alcohol is the worst to trigger fatigue, and contributes to irritability. When we go for a jog or a walk, this helps best. There are of course times when we don't feel up to going for that walk, but even if you do 1/2 the time and distance, it is better than just lying around in the same position.
In your sister's case, she may be lacking in something, some food intake (sugar, B-vitamins, iron, protein...etc.) perhaps. If she is already seeing a specialist for HD, I would also recommend a doctor who specializes in "orthomolecular" medicine. It focuses on vitamin and mineral intake rather than pharmaceutical drugs.
Also, a change of scenery is good once and a while, but I find that too much change seems to cause more exhaustion.
Take a short drive somewhere, go to a different park or shopping plaza, or whatever she is, or used to be, interested in.