I think the wording in the will makes your aunt's intentions very clear. Your rights, while Y is still living, are close to zero.
Google on "without impeachment for waste" and you will find that so long as Y refrains from positive destruction of the property, you have no right to interfere. She can neglect it, and you can do nothing about it. The fact that somebody cuts the grass (presumably at her behest) indicates two things:
1. She is not following a course of wanton destruction.
2. She is still alive.
As Y is elderly, it would be in your interest to have a way of knowing if she dies. In your position, I would try to make contact with the person who maintains the garden to see if you can open a channel -- not necessarily to communicate with her, but simply to have some possibility of knowing if she dies.