I am sorry to learn of this situation.
If I understand correctly, your Niece is the one that is buying the home, but you are the one that is identified on the contract as the "buyer."
If this is correct, you do not have a way to legally force her to assume the contract (or to force the seller to accept the change in buyer), however, you do have the right to exercise your rights as the buyer. This means that depending on the status of the lease to own contract, you can terminate the lease and revert the home to the owner, you can purchase the house outright (get a traditional loan) and sell it, or you can renegotiate a loan with the seller.
Keep in mind, none of this is going to be risk free - your niece is likely to claim "breach of contract" against you - this claim is going to be weakened due to the fact that the writings all support your property interest, but I just want to alert you to the risk.
These matters are usually best resolved through mediation (especially given the fact that this appears to be a family dispute). - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.