Sigh, how old is she?
Let's see....let me pull from my child resource bag some things you can do to avoid a 'battle' with someone else's child. You really have to be very good at maneuvering around children who have extra doses of energy and/or are prone to tantrums and non-tantrum bursts of emotion.
First of all, as you probably know, you may be the one 'steady' in her life. So....of course it's so important that you stay sane (though maybe not always possible
Do you have a play area out doors? I've always found that the absolute best solution for high strung children is to just have them outside as much as possible. The best possible solution for them would probably to have a tent out there and just let them be out of doors all the time. If you do, you can take any work you have to do out there and just let her RUN. My own daughter always comes in from the outdoors very tired, but also centered and happy. Children need more outside time than we know and often they just don't get it, because our society is no longer outdoor friendly.
Secondly..enlist her help on anything and everything. Children love to be helpers and it keeps them occupied with things that are constructive rather than destructive. Even small children (three and up) can work with a not very sharp knife cutting veggies for dinner if supervised and shown the correct way to cut. Setting the table and loading the dishwasher are also options. Once she gets to see that she's being a positive force for 'good'
, she'll want to help in other ways. Folding laundry, dusting shelves...anything you can 'hook her' with will help both you and her. Also, given her current situation, making cards for Mom and Dad...pictures for Mom and Dad....work that in whenever you can. That might open things up for her to express to you (a non-partisan third party) what she feels about what's going on, and then you can just listen and care.
These things that I'm mentioning will help her to be part of your community. Also, depending on her age, any messes she makes she should at least have a significant part in cleaning up. This is a direct consequence, which will be more effective than time out (those do help in a pinch), because she sees that she made the mess, she cleans it up. She may not be as apt to do that next time. I had to do that with Madison last week, she actually made poka dots on the wall with a marker. It took her two hours, a sponge, some warm water and a few tears to clean it all off. But I didn't help her. Nothing else could happen until she'd done it, and it provided a terrific lesson experience for her.
Anyway....good luck. Sounds like you have your hands full. Feel free to pm me if you want to talk about it.