Mr. Selcuk Argun seems to have explained almost everything needed in response to the questions in your first message, however there is something I'd like to add in order to contribute as starting from the beginning.

There's a point that must be first clarified regarding whether Turkish law will be executed on this issue or not. Ordinarily, the national law rules of a deceased person gets executed on his/her inheritance even if/she passes away in Turkey. However, the inheritance transactions and claims concerning an immovable/real estate in Turkey are excluded of this rule contradistinctly ordering that Turkish law rules shall be used.

It can agreeably be reckoned that Turkish civil code's provisions will be executed on your issue in terms of the short explanation above.

Regarding Turkish inheritance law, you should first know that the spouse of a deceased person -really- acquires a quarter(1/4) of the total assets left from him/her whilst the rest assets get equally shared by the children(unless the otherwise specified by the deceased person with a will or testament before he/she died). What your step father told you is, consequently, true in respect of the Turkish Civil Code's related provisions.

The second point which is necessary in this issue is the enforceability of the will that your mother left in the UK. Turkish civil code permits wills or testaments left in a foreign language to be enforced in Turkish courts and bailiff's offices provided that it's translated by a sworn interpreter. However, you can not exclude your step-father from this property due to 'reserved portion' institute specified in Turkish civil code even if you can get the will executed in Turkey. The whole inheritance portion of your step father is reserved as it's ordered in Turkish Civil Code unless he committed something to be exheredated by your mother(such as killing or attempting to kill her).

I made some explanations regarding 'reserved portion' institute in Turkish and Continental- European law systems under other topics concerning inheritance questions. You can take a look at those topics to know better what it's meant to be, and ask again here if there's still something needed to get clarified in your mind.

I hope this helps.

King regards