**Answer:** There are four boys and six girls in the family.

If there were B boys, and G girls we know that Daniel has twice as many sisters as brothers, so 2 x (b - 1) = g.

And his sister, Jessica, has one more sister than she has brothers, so g - 1 = b + 1.

2(b - 1) = g [1]

g - 1 = b + 1 [2]

Using [1] in [2] gives:

2(b - 1) - 1 = b + 1

Expanding the bracket gives:

2b - 2 - 1 = b + 1

Simplifying the left hand side gives:

2b - 3 = b + 1

Add 3 to each side gives:

2b = b + 4

Subtract b from both sides gives:

b = 4

Using b = 4 in [1] gives:

2(4 - 1) = g

6 = g