We had a few correct answers for this challenge. Firstly Alex from Branchburg Central Middle School in USA said:

There are seven children in the Brown family: three boys and four girls.

Julian from the British School Manila in the Philippines, gave this mathematical explanation:

Number of male children: b
Number of female children: f

b=f-1
f=2(b-1)

b=f-1
f=2b-2

b=2b-3
b=3

f=b+1
f=4

b=3. f=4

7 children

From Moonstone at the Hammond Academy we had this:

We think that there are 7 children in the Brown family altogether. Sally has 3 sisters and Mark has 2 brothers.

Finally Katie at Raysfield Infants School, wrote this excellent explanation as to one way of solving the challenge.

We read the instructions and logged on to the website. There were three 'questions'.
Q1. Sally had equal numbers of brothers and sisters.
She did not have an equal amount of brothers and sisters so we made it equal by adding a sister.
Q2. Mark has twice as many sisters as brothers.
He now had 2 sisters and no brothers so we added a brother to make Q2. Correct.
We checked Q1 again to see if it was true. It was not so we added another sister to make Q1. correct.
We checked Q2 to see if this was correct still, it was not so we added another brother to make Q2. correct.
We went back to Q1. To see if that was correct, it was not so we added another Sister.
Now Q1. is correct, we checked Q2. and that was correct too.
We then added up all the children to see how many there were.
There were 7 children.

Thank you for those results of your work, well done in persevering.