My girlfriend and I (both of us are on the lease) allowed her aunt and uncle to stay with us until the house they were building was completed (was told it would only be a month or maybe two). That was 4 months ago and come to find out there is no "house being built". We no longer want these people living with us, yet have 3 months left on our lease. They say we have to evict them. They did not pay rent last month and we're now half way into next month's rent. How do we get rid of them

You have to give them a written notice that their tenancy is terminated. You didn't say what state you're in for the proper time but you should be safe giving the 30 days notice.

If they haven't moved out within the 30 days then you have to file a lawsuit for eviction.

(You have the option of giving them a pay or quit notice instead which has a much shorter time but if they pay in full before the court date then you can't evict them.)

I suggest that you move on this immediately. If your lease ends and you move out and they stay you will be responsible for the rent and any damages until they are gone.

I might be inclined to warn the landlord that this is happening and what you are doing, to show your good faith efforts - not that he or she has to feel for you if the eviction isn't final when you leave.