"女校男生" it is not a problem. Many DSS is Girls more than Boys.
I support your wife, due to younger sister is not confirmed to admiss same DSS school.
School fee is small issue or non material issue.
I will consider the listed issue.
The standard of 蕭小 is better or upper average.
Travelling time is long or not, within 30~40 min for single trip.
Time arrangment for same school is better than 2 school, due to DSS is admissed by interview.
Time arrangement means school events, school hour, calender, ECA & etc.