I met a bipolar hoarder in group therapy once. I'm not exactly sure how early in her life it began, but she had been doing it for quite some time. She had lost a lot in her life and the hoarding seemed to be her way of holding on and controlling in a world that felt out of control to her. She also had severe OCD.

I haven't come across any information to support hoarding being a symptom of bipolar disorder, but I agree that the compulsion to buy is amplified during times of mania. The person I met had even less ability to reason over her purchases when manic and it was difficult for her to even see it as a problem. She became agitated when we discussed it in group and felt strongly that it wasn't her fault even though she admitted it was a problem. She was medicated for her bipolar symptoms, but the hoarding persisted. I'm not an expert, but I think you're dealing with two separate diagnosis that affect each other. Is your sister on meds?

I hope others here can shed more light on this for you.