Interests in properties can be held in only two ways - tenants in common, where people can have unequal shares (as here) and those shares pass according to a persons will when they die OR joints tenants, where peoples shares are indistinct to each other and they pass automatically to the remaining joint owners.
You should check to be sure you hold as tenants in common.
You can check by downloading a copy of the register for the property from the Land Registry by paying a £4.00 fee:-
Look at the proprietorship register of the title you have downloaded from the Land Registry (where the proprietors of the property are listed). If the following entry is immediately below the named registered proprietors then the interests are held as tenants in common:-
" No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the Court"
If there is no such entry then the property is held as joint tenants.
Your sister would not acquire any greater share than she already has simply by virtue of her occupation of the property, this does not affect your interest at all.
If you wished to realise some equity from the property then you would have to ask that either one or both your father and sister make an offer to buy you out based upon the current market valuation of the property. If they are unwilling to make an offer then you could apply to Court for an order of sale so that the equity is realised and your receive some money. Obviously, this involves litigating and therefore incurring legal fees so if you wished to realise your equity then it would be more cost effective to resolve it informally with your family subject of course to you receiving a fair deal.
Again, it's very important to make sure you do hold your interests as tenants in common, if you do not then the provision in your father's will as to his share will not be legally enforceable.
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