From the way you describe the situation, this doesn't sound like a JW/WT problem. Treating her elderly parents poorly means she's treating both of them poorly, not just the unbeliever.
Depending on the severity and type of maltreatment, this could be a matter for the civil authorities. Check the listings for your parent's area and find a government entity with a name like "Elder Abuse". Call them and discuss your concerns. They will probably do a well-being check, just a conversation with your parents to get their views on the relationship and to gain insight into your parent's current abilities to handle their own lives. They will not do this in your sister's presence (or yours!), so your parents will be able to speak as freely as they choose.