Just some insight based on my own experience....
Sometimes when I feel overloaded with things going on in my life, I might sound the way you describe your sister as sounding. I personally am fine but there is a level of stress that is just exhausting to get through. I just don't have the energy I normally have & I am sure it comes through in how I sound.
But that said, people can also hide how they really are not wanting to share details & I am sure I could be that way too.
Not always easy to distinguish but I think your perception that something is going on is probably pretty accurate.