Joint ownership

My wife and her sister, both UK citizens own an apartment in Side. My wife's sister wants to sell the apartment whilst my wife wants to keep it. In the form or compromise they agreed to sell after October this year. My wife's sister has now reneged on this agreement and says that if my wife does not sell before then she will withdraw her co-operation and not allow my wife or her family to use the apartment. My wife pays all the maintenance and insurcance for the apartment as her sister refuses to pay anything.
Can my her sister lock her out or restrict her access to the apartment? Can UK law have any influence in this matter over Turkish law?

Can my her sister lock her out or restrict her access to the apartment?
Definetly, she cannot do such a. Each of the co-owner has the equal right use the co-owned properties. In any case, if your wife is excluded from using the co-owned prperty, she is entitled to sue her sister on grounds of trespassing.

Can UK law have any influence in this matter over Turkish law? the answer is no. The apartment is an imovable property, a real property. The law will be applied on the matter is Turkish law. However, as you must be aware that a co-owner using the co-owned property is liable to pay a compensation to the other co-owners. And it is true for the Turkish law to some extent. İn Turkish law, to be entitled to such a compensation the excluded co-owner mush make a claim.

On the other hand, your wife sister does not have to obtain your wife's participation to sell her share in the property which is not possible in English law due to joint tenancy.

or the sister may take a legal action to terminate the co-ownership. In such a case the court will decide to auction the apartment and give the purchase money to the co-owners. If your wife and her sister agree that only the current co-owners can join the auction, the court will excluded the other possible purchaser into the auction.