Prohibition for the wife of the deceased brother who has no children to marry an outsider.
“The wife of the deceased should not go out and marry an outsider.” Devorim 25:5
Should the deceased have a brother from the same father who has not first done Chalitza (+46) and if she married and outsider and they had intercourse they both violate this prohibition.
She must get divorced and is required to have Chalitza performed by the deceased brother and then she is permitted to an outsider.
This applies in all places and all times.