It's admirable that you're taking over in caring for the dog. Your sister & dog are lucky to have you.
Make sure you build a bond and obedience train her well. If she takes you seriously & trusts you then it will be easier to ask her to be quiet, lay down, stay, ect WHILE having her listen.

If you can first start by some how tieing her down in what will be her specific corner or "place". Make it dog friendly & comfy for her. You will work on getting her to learn to "settle" there without other people being around. Get her used to it & the behavior you want. Give her toys that can keep her entertained for long periods like a kong & such.
Once you feel she's getting used to her role & is more reliable, start doing the same thing with other people there. The more consistent you are in redirecting behavior you don't want, the less she will test you. Once she gets into the habit of knowing what she's expected to do, you can stop tieing her down & continue reinforcing what you want from her.
OH!..>.forgot to mention...tire her out very WELL before you take her to the dance class EVERY time. If she has a lot of energy left to burn she will be harder to focus.