sounds like you are trying to the right thingI think taking away her phone and canelling her outing are fair enough. As for getting up bar physically forcing her there isnt much you can do. I wouldnt flame the attention seeking. If she is hugry or thirsty or bored she will get up, if you have wireless internet I would consider changing the password until she comes out and has a proper discussion with you so she doesnt spend the entire day in her room on the net. Im sure she will get bored at some point and will venture out

I think you need to have a talk to your dad with your DH present and agree about what boundaries you are setting and suitable consequences, if you plan on having her stay. You all need to be on the same page to help make your lives easier and to reduce the incidence of her manipulating everyone involved. You then then need to discuss these with your sister

As for clothes I would be contacting centrelink and seeing if you would be entitled to receiving any assistance for having her in your care, if you dont due to your Dad's income then I would be discussing a child support type system with him where he pays for his daughters needs. My IL's have raised my niece and had a similar situation.

If her presence in your home is having a negative affect on your family then you need to carefully consider what impact her staying will have on your family short and long term. And I agree a reliable form of contraception would be a major concern wherever she is staying, you dont want to be raising your sister and your niece or nephew!