LouLou, I think that the link from bluesfan is excellent - it is well worth a read.

Geneticists use the word "penetrance" to explain how likely it is that inheriting a particular form (an "allele") of a gene leads to a health problem. In the single-gene diseases that I mentioned above, penetrance is often high (close to 100%) but these diseases are very much the exception, not the rule.

In most diseases inheriting alleles of many different genes are usually risk factors. This is very complicated; for disease X inheriting allele 23 of gene A might be a negative risk factor. This might be made worse in somebody who has inherited allele 45 of gene B but better if they have inherited allele 14 of gene C, and so on.

I don't know any examples in which sorting out these complicated patterns has led to improved therapy/treatment but this could change.

Now, cutting to the chase for you and your sister (all that I am concerned about here), you wrote:

"The neurologist she has just seen has told her that it is pointless going to get the biopsy done as there is nothing they can do about it as it will be hereditary."

Bluntly, this is nonsense. The neurologist may or may not be a good clinician but s/he knows slightly less than nothing about genetics.

It is what is called genetic determinism - "OMG, it is in my genes. There is nothing that I can do. I am doomed.". With the rare exceptions of single gene diseases, genetic determinism is a total myth.

One-line summary; if you (with confirmed SFN) or your sister (with possible SFN) decide to have children then go for it. Your genetics (and those of your partners of course) mean that your children are unlikely to be at greater risk of SFN than any other children.