You can state something like this:
As we now are facing a protracted legal conflict over mother's will I would like to make the following proposal/offer to settle all claims surrounding mother's Will.
In exchange for your dropping any challenge to the Will I propose the following
1) Sell mom's property located at and split the net proceeds equally between us;
2) We equally share the costs and expenses associated with Mom's funeral
3) We equally share the expenses associated with administering the estate including but not limited to attorney's fees, and any other fees associated with the estate as required by law
4) You will reimburse me for half of the expenses I have already y paid on behalf of the estate including but not limited taxes and bill for which I will provide written documentation supporting the expenditures.
5) You agree not bring or continue any Will contest or other legal action concerning the estate or administration of the estate.
If the foregoing terms are agreeable to you please let me know you accept this proposal by signing at the bottom and returning to me.
I hereby agree to the terms as set forth above. I understand that this represents the full agreement between the parties.
include a signature line for you and you sister along with the date signed.
You should use the above only as a template. It is not intended to be the exact form for your agreement or situation.
Please accept my answer with a rating of 3 or better so I may get credit for my response. If you have follow up questions please ask. Please note that paying the deposit does not cause funds to be disbursed until you rate my response (3 or greater).