Jack is a boy; Jill is a girl   (If to follow to commonly accepted way of using English names).


Let b = number of boy children in the family

    g = --------- girl  - - - - - - - - - - 


The condition says

    g-1 = b         (1)     ("Jill has as many brother as sister")

    2*(b-1) = g    (2)     ("Jack has twice as many sisters as brothers ")


From equation (1), substitute  b = g-1  into equation (2).


    2*(g-2) = g

    2g - 4 =  g    =====>  g = 4.


ANSWER.  There are 4 girls and 3 boys in the family. In total, 7 children.