OOOOK, it sounded like they were all on the 5 acre parcel. For the time being, if you can, I'd make them the only 2 horses somewhere, either on the 48 or on your property, I'd get them away from yours and your sister's horses. I'd also send them a letter, certified mail, return receipt, that as of a certain date if the horses were not removed before this date that a/they could start paying board (make it HIGH to motivate them) and b/if board was not paid by such and such a date, I'd consider the horses abandoned and would then sell them to cover expenses for their care.
If it was me, I'd write them something along these lines:
As we discussed last year when you brought your horses to me for care, on Nov. 1, 2011 (or whatever the correct date really is), you were to build fencing on your property so that you could care for your horses properly on your own. I agreed to do this at no charge to assist you in being able to purchase the needed fencing materials, so that you could remove your horses from my property and to your property into your care, in a reasonable amount of time. At no time did we agree this boarding situation was to be permanent.
Since it is now July 2, 2012 and you still have not removed your horses, and they have caused damage to 2 of mine, I must now build fencing on MY property to house your horses. I will bill you for the costs of labor & materials upon completion of said fence. The professional estimates I am getting for this amount of fencing are: $XXXXX and you will need to pay upon completion of the project.
As of August 1, 2012 I will also need to be paid board for your horses, put their names and registration numbers here, in the amount of $XXX per horse for a total of $XXX per month, due and payable on the 1st of every month for the time they are remaining. I am enclosing a board contract which must be signed and returned with your first month's board payment or your horses must be removed from my property before the 1st of August, 2012.
If you choose not to pay board and leave the horses here after August 15, 2012, I will consider them abandoned and will sell them to recoup the costs of damages I have suffered since their arrival.
That puts everything on a business footing, removes any personal issues from the transaction and reminds them, in a kind of nice way, that they are being deadbeats and you are not going to take it anymore.
I have a board contract that I wouldn't mind you using, as long as you took out all of my info and made it into yours. My contract automatically puts a lien on the horses if the board isn't paid and has a release of liability in it. You'd need to check TN for specifics required in the release, but the contract is pretty cut and dried and should be good. The whole idea is to get them to move their horses anyhow, NOT put you in the boarding business.