The term ataxia itself means an abnormal gait which most of the time seems to be similar to that of an alcoholic. When your Dr indicated Spinocerebellar (SC) it means that your problem is most probably being caused by problems or lesions in these areas of the nervous system.
I am personally a parent of a child with this form of ataxia. As we have never had any cases of SC Ataxia in my country we were sent to UK 3 times for the necessary tests. Being a health care professional myself I demanded to be informed on each and every test my daughter was going to have. When I was told about the test you are mentioning I was skeptic, and did my research very well with regards. What I found out is that it was the correct price not only in UK, but also in the United States of America, and other EU Ataxia centers. I also found out that we were very lucky to have this test done as most of the Medical practitioners never prescribe such testing quite obviously due to its expense.
I also found out that those officially diagnosed through laboratory testing are being listed in a rare disease register which is being kept in the United States, in case new medical breakthroughs are identified. This alone is already good enough reason to be tested officially.
I can understand fully your concern but the only thing I think about your doctor is that he could have explained to you better your position and the reasoning behind his decisions. I would like to point out that I would never presume to judge in any way your Medical officer or the care he is giving you, but do your research well before deciding to look for second opinions. These can be very expensive and you would still be possibly receiving the same treatment. I was going to look for second opinion myself for my daughter, but when I found a friend doctor who explained to me my daughter's situation I found out that my insecurities were only based on my lack of knowledge on the subject matter. I don't blame myself or anybody for this as I have already expressed that these cases are considered as very rare. In fact I was told that even in this state of the art neurology center were my daughter was sent, it was their first case of SC Ataxia.
What I suggest is that you find a health care professional (friend) you can trust and discuss your case properly before you incur other expenses. Use your financial resources in best way possible, maybe you might find that spending money on occupational therapy or devices that can help you in your daily activities of living is a much wiser idea. I feel that I can understand this situation quite well, as even though all medical expenses and tests (including traveling expenses) are being paid by my national health care system, I still had to pay a lot of money to convert my residence for our daughter's progressive needs.
I would also suggest you research the condition your uncle has from reputable healthcare websites. You might find that there might be some particular signs and symptom or genetic situations that might not relate to you. For example it might be a condition that affects only males or only at a certain age or life situation: I know nothing about the condition you mentioned, but what fits Patient A might not necessarily fit Patient B even if they are related. This might be the reason why your Doctor disregarded this when you told him about it.
If you would like to know a basic knowledge about Ataxia, The University of Minnesota Ataxia center published a book free of charge on their website. It covers all forms of ataxia one by one, and gives information about signs, symptoms and available tests for each and every one respectively.
I hope I was of help for you and for those reading these forums, if you need further help feel free to contact me even by email if you like.