What if they "do something about it" and it turns out no one wanted their "help" and they alienate both their aunt and the "slave"? That would be the obvious question on my mind. Especially if you do something irreversible like call the government to intervene.
This is a fine-edged and very delicate situation. That is why I encouraged the OP to seek legal guidance. The first step isn't to call in the government, but to gain access to advocates that can work with his/her family to separate fact from fiction.
With outside counsel (legal, psychological, or both) involved, if the OP has misunderstood the nature of the family members' relationships, it will be revealed. On the other hand, if the situation is abusive, having an objective third party there for support and advice will be extremely important.
I am not trying to downplay the emotional difficulties present here. Fearing that you may alienate portions of your family is entirely understandable. But not acting means you are complicit.
If you see someone who is in trouble and who needs help, face your fears and work to build the support structures that will return to them their "voice" and personal agency going forward.
