Laws vary world wide ... check your local law books or consult with a lawyer ... here in Canada, we have 'gift tax' and 'inheritance tax' for things of high value ... here, the way around that is to either have her set up a trust/trusteeship with the property an item in the trust/trusteeship and have herself as the person who controls the trust/trusteeship, and then, in her will, she just names her replacement as controller of the trust ... in this way, it is the trusteeship that owns the property, not the person ... it is like a corporation, it is a legal entity ... all that changes is the name of the person who controls it ...
... or, another way is for her to sell half ownership to you for a single dollar before she dies, then when she dies, the surviving partner automatically gets full ownership ...