Advice please

My sister and her fiance (of 4 years) have partial custody of his 4 year old daughter. She switches between their house and her grandparents house (where her biological mother, her boyfriend, and new baby live <1yr.) every week. There are no rules at the grandparents house. No bedtime. She does whatever, whenever she wants. They buy her presents when she demands them and even gives her presents every week when she comes back. She now expects a weekly gift.

My sister and her fiance have rules. She has a set bedtime, don't talk back, clean up after yourself, respect others...the important stuff.

When she goes to their house as soon as she sees them she says "Did you get me a present?" "Where is my present?" When they tell her she doesn't have a present her response is usually "I'm going to wreck this house if I don't get a present." "I hate this place" "I'm running away" and the most disturbing one "I'm going to get a gun and shoot everyone in this house if I don't get my way." Word. For. Word.

She doesn't watch anything violent at my sisters house and she's never been exposed to anything violent at least not at their house. She goes to an at home day care that my sister knows nothing about. The grandparents refuse to give them information or even an address. They don't have access to her medicaid card so they can't take her for therapy, which I believe she desperately needs. The grandparents won't even entertain the idea because they don't see anything wrong. They've gone through lawyers and court and this is the set up they say works in her best interest. Clearly not. All of this has been going on for 2 or so years. They're at a loss and its greatly effecting their relationship with each other and the child. They've done time outs, taking away privileges and toys, spankings (don't agree but ..) can anyone offer advice??

(Backstory: father and mother split up, mother was gone and not in her life for about 2 years, he sent her to live with grandparents because they could provide what he couldn't and once he got a stable job and home they refused to give her back which is when they went through court. No drugs or violent past)