Neurological disorders are sometimes hard to diagnose until (sadly) they get worse.
My OH has had PD for at least 10 years - he did not start with stumbling. He had an obvious tremor in his left hand.
One of his earliest symptoms was the inability to write clearly and this is often the first thing to appear - the poor writing is not to do with the tremor (he is right-handed) but something more complicated going on in the bit of the brain involved in this particular skill, which is of course related to fine motor skills. But writing seems to be something that gets clobbered early, even when other fine motor activities are OK.
Falling has not been the most obvious feature - he has only fallen about 4 times in all those years - unfortunately the last fall fractured his neck of femur and landed him in the helpless state in which he now is.
There are variants of PD and different people have different predominant symptoms. It is a complicated illness and has a lot of other problems associated with it rather than the easily recognised tremor, e.g. gut motility is also affected with severe constipation being common. Anxiety can also be a troublesome feature.
I am sure that the neuro bods will have had PD in mind as one of their possible diagnoses.
Has your relative asked if it might be early PD?
We had no problem with the diagnosis as OH is a doctor and he had known for ages what it was, and chose his moment to see GP at the point when he thought the benefits of treatment might outweigh the potential side effects.
What a worry for you. I hope that some diagnosis will be forthcoming soon.