The confusing thing about these disorders is that there is wide variation in symptoms among family members, even with the same disease. The reasons for this are not understood.
They are all complicated diseases.This is why I encourage people with inherited SCAs to be examined at an ataxia clinic, which sees lots of cases, rather than just a few like most neurologists.

Unless your father also had an SCA, the SCA7 gene had to come through your mother. SCA7 is autosomal dominant... it isn't passed in a hidden fashion as are recessive-inherited disorders.

Hopefully, you familiar with the National Ataxia Foundation. If not, I would encourage you to join. Also, you might want to investigate the possibility of your affected family members signing up for one of the databases being developed. The researchers are trying to establish a database of those with definite diagnoses, so (1) they can compare and keep records of symptoms (2) have a ready population available for clinical trials as treatments are developed. There's one through UCLA and one through Emory U.