They are 64, 61 and 59.

An even number of years must have past between the two scenarios, since at least one of the sisters must have had an odd age in the earlier scenario (there is only one even prime). So therefore the cube numbers must be of the same parity. They can’t both be odd, since 1 is not bigger than any prime numbers, and 125 exceeds a human lifetime, so 27 is the only viable odd cube. So the older sister must have been 8 in the early scenario and 64 now.

There are four prime number below 8: 2,3,5 and 7. Adding 56 years to these gives 58, 59, 61 and 63. Only 59 and 61 are prime, so these are the younger sisters’ ages now.