What can I do legally when a person won't move out of my apartment when they are no longer on the lease??

Me and my gf live w/ her sister who was set to move out at the end of this month...she had a place lined up that she really likes. So we were all happy and she is no longer on the lease come May. But then she found a slightly better place two floors up and now TELLS us that she is staying at least another two weeks to a month so she can move into that place instead. Might have been ok if she asked/begged, but she showed us no respect and we have a ton of all new furniture coming in on May 3rd and all her stuff will still be here so it will be a big mess. There has already been huge fights between my gf and her and she refuses to stick with the apartment she originally had (which is same floor plan just 5th floor instead of 7th/Top). Somebody please help us and let us know what we can do legally to get her out b/c I can't have someone disrespect me like that and tell me that we need to change our plans to accomodate her selfishness. What can we do????

She may not be on the lease as of May (if the landlord took her off and you signed a new lease), however as long as she still pays her portion of the rent, she is considered a month to month tenant. She doesnt have to beg or even ask you to stay. She has residency. She just changed her plans. That happens. And as long as the landlord allows her to stay over so she can get the apt on the top floor, then there is nothing you can do about it. Its the landlords decision. Did you tell her you were ordering new furniture? Sounds to me like it was her apt to begin with and you were living with her, since its her stuff there. She has the upper hand here as the lease is in her name and the landlord has the legal relationship with her. The landlord is not going to force her to move. He will just make her a month to month until the new apt is ready.
Sorry. You cant do anything legally to her. And its not YOUR apt if her name is on the lease and the landlord is letting her stay on a month to month basis after May 1.
Find out if there is a storage unit on the premisis that she can move her furniture into until she moves. Or you can delay the delivery of your new furniture or put it in storage.
Sorry, you cant force her to move.

if she's not on the lease and she is in your appartment without your permission then she is legally trespassing, all you have to do is call the police

Take it to Judge Judy.

She clearly has established a residency at the rental, if she stays past the end date of the lease, or stays after proper notice is given to terminate a lease, she is then labeled a hold over tenant

either way, only the sheriff office with a court order eviction can remove her from the premises, any action on your part is called self help is illegal and will cost you big bucks if you do something personally to lock her out