Rules is a hard one, you don't wanna be too over the top and make rules she can't possibly abide by or be more lenient on her than you are on your own kids. Would she be expected to help out iwth cleaning and cooking?

Maybe just explain to her that while she lives iwth you she'll be treated exactly the same as DD, all the rules that apply to DD apply to her as well. Work out whether she'll be returning to school or getting a job and make her stick to her decision to do either one of those.

I have no idea what to advise on board, I'm sure you can both come to an amicable agreement on that, but I do agree she should pay her way towards food and bills.