How to deal with a spoiled teenager?

Wow, that brat is really unappreciative. First, you may want to let your fiance know that the situation is stressing you and that you would like to have a talk with his sister. Ask him to stand in agreement with you so you both can combat her together. You don't have to make it seem as if your ganging up on her, however let her know that everybody else is pulling their own weight around the house and it gets tiresome for you all to keep babying her. She's old enough to do chores for an allowance. Even if she doesn't clean the entire house, she needs to pick up after her own mess. I hope you can have a non-confrontational talk with her and, IO hope your future family stands behind you. Goodluck!!!