First you have to know that Tay–Sachs disease is an autosomal recessive genetic disorder. Below we will assume that the mutation is at low frequency in the population.
**Probability that Susan's maternal grandparents carry the mutation**
The probability that Susan's mother carry the mutation is $\frac{1}{2}$. The probability that the mutation is present the Susan's maternal grandparents is therefore also $\frac{1}{2}$.
**Probability that Susan's aunt received the mutation if her parents where carrying it**
Now, if the mutation is present in Susan's maternal grandparents in a single copy (which is a consequence of the assumption of the mutation being at low frequency in the population), then the probability that the sister of Susan's mother received this allele is $\frac{1}{2}$ too.
**Putting these two probabilities together**
Putting these together. $\frac{1}{2} \frac{1}{2} = \frac{1}{4}$. The probability is $\frac{1}{4}$.