Hi,

Turkish law says that Turkish Law will be applied to the conflicts between the heirs, if the conflict is based on real estates that are in Turkey.
According to Turkish Civil Code your step father will get 1/4 of your mother's %50. So you and your brother can get 3/4 of %50 posession of the real estate.
but if your mother's will does not especially exclude the properties in Turkey, than British law will be in force and your mothers will be valid.