Your sister sounds like she is extreemly selfish. I think you should just stick with the plan of what Mom wanted done with the items your sister is seeking, the John Deere goes to whomever Mom wanted it to. This way you are honoring your Mothers wishes and spirit and it alieviates you from the decision and burden of having to make a decision.
While I understand this is your sister, blood or not, sometimes you just have to cut ties and let people go. It is not always the easiest thing to do but sometimes the best form of self preservation as it sounds like she is causing you a lot of inner turmoil. When she is ready to be responsible in all matters, stop being so selfish and seeks you out then you can allow as much as you are willing to accept.
I think once you stop worrying about her and let her go you will find you really don't miss the person she is being at the moment and you will be lighter for having let the burden go.
Good luck and be strong.