There are different sizes of suitcases. If you would want to emphasize that your sister's suitcase/bag is really big/large then we would say, "My sister's suitcase/bag is huge.
Grande usually means big or large. To say huge would be something like "A mala da minha irmã é enorme/gigante/muito grande". :]
Why is "my sister's suitcase is heavy" wrong? I thought "grande" was also heavy?