If I had one more sister I would have twice as many sisters as brothers. If I had one more brother I would have the same number of each.
How many brothers and sisters have I?

Three sisters and two brothers.
This can be solved by simple deduction, but if algebra is used let x be the number of sisters and
y the number of brothers:
x+ 1= 2y
y+ 1= x
Therefore, y + 1+ 1= 2y
so y =2
or x + 1= 2x - 2
so x = 3