If m + n means m is the sister of n, m − n means m is the brother of n, m × n means m is the daughter…

Three persons are definitely female. In $a + b$, a is the sister of b, so a is female; in $b - c$, b is a brother (male); in $c + d$, c is a sister (female); in $d - e$, d is a brother (male); in $e \times f$, e is a daughter (female). So a, c and e are female and b and d are male, while f is only a parent of e and could be either. The number of persons shown to be female is therefore fixed at three; counting f gives four, which the expression does not support.