Erect ears stand up via cartilage and yes, if it's up, it's up. They MAY droop with age, but for the most part if it's up at 6 months theres a good chance that her current ear set will remain that way. In my experiences, the ears will only stand more erect with age and not droop IF they are going to change at all.

http://www.shalakoaussies.com/BreedStandard_Ears.html

Has some information on the correct ear sets.

Jamie has close to the ear set in the second photo but her's fold higher, whereas Charlie's ears are both fully erect. At 4 months one was fully erect and the tip on the other slightly flopped over, but both stood erect with time.

Your SIL shouldn't worry so much -- the dog will still love her the same with or without a "proper" ear set I think one erect and one floppy ear are ADORABLE!!!