LouLou, I am puzzled by what you have been told.

In general, genes which are strongly associated with a disease come in two classes.

In one class the mutant gene encodes a protein which has gained an abnormal (pathological) function. These diseases are very rare and show a strong familial pattern - a child of a parent with one of these diseases has a 50% chance of inheriting the mutant gene and so will get the disease. This does not seem to apply to you and your sister.

In the other class the mutant gene encodes a protein which has lost its normal function. Somebody with one copy of the mutant gene and one normal copy will be fine because the normal gene copy ensures that they have the normal form of the protein. People like that are "carriers" of the disease. These diseases are relatively common.

Cystic fibrosis is an example - somebody may develop it if both of their parents are carriers. Cystic fibrosis occurs when somebody inherits the mutant form of the gene from both of their parents - there is a 25% chance of this happening.