“I live with my father, my mother died almost two years ago and their joint will states that, in the event of my fathers death, all monies and property is left to me. My father has now asked my mothers sister to move into the property, what are her rights as to tenancy and claiming half the properties value?”
Well, there’s a very old saying in the law: “Nemo est haeres viventis” (“no one is the heir of a living person”). While your father is alive, his property is his, and he can do with it what he pleases, regardless of what’s in his will. This makes sense, since he can also change his will any time he wants.
So, if your father owns the property and sells or gives it to your aunt, there’s nearly nothing you can do about it (unless, for example, he’s legally incompetent).
But let’s go back a moment to your mother. Since this was a joint will, there may be provisions in there which are locked in now that your mother has died, at least to the extent of her interest in the property. In other words, your father may not be an unrestricted owner of all of the property right now. The only way to determine the nature and extent of this would be to review the will, and possibly the probate proceedings that took place after your mother’s death.