Twins—two children born at the same birth (from the same mother); can refer to the pair regardless of sex.
あの双子は見分けがつかないほどよく似ている。
Those twins look so alike you can't tell them apart.
彼女は双子の母親で、毎日忙しく子育てをしている。
She is the mother of twins and is busy raising them every day.
双子の間でも性格や趣味は全く違うことが多い。
Even among twins, personalities and tastes are often completely different.