If a family has boys and girls and some boy has 2 more sisters than brothers, 
then the number of girls in the family is 1 more than the number of boys,
and each girl in this family has the same number of sisters as the brothers.


If a family has boys and girls and some girl has 2 more sisters than brothers, 
then the number of girls in the family is 3 more than the number of boys,
and each girl in this family has two more sisters than the brothers.


From this reasoning, we conclude that the child who speaks and tells the story is a girl.


So, in this family the number of girls is 3 more than the number of boys.


Since the problem mentions about the oldest brother, the context is that 
there are other boys (more than one) in the family. 


Since the oldest brother does not count himself when he counts his brothers,
we conclude that the oldest brother (as any other boy in the family) has 4 more sisters than brothers.


ANSWER.  In this family, the oldest brother has 4 more sisters than brothers.

Since you're probably an algebra student, maybe you'd like to see a solution
with an unknown or two, and not just basic arithmetic.

Suppose the siblings consist of b boys and g girls.
Since nobody can be their own brother or sister,
each boy has b-1 brothers and g sisters, and
each girl has b brothers and g-1 sisters. 

If I have 2 more sisters then brothers...

If "I" is a boy, then he has b-1 brothers and g sisters, and g=(b-1)+2, or g=b+1

...and each of my sisters also has 2 more sisters than brothers...

But then each of his sisters would have g-1 sisters and b brothers, and we would
have g-1=b+2 or g=b+3, not g=b+1. So "I" is not a boy. So "I" is a girl.

Then she and each of her sisters have b brothers and g-1 sisters, and
g-1=b+2 or g=b+3

how many more sisters than brothers does my oldest brother have? 

Each of her brothers, including the oldest, has b-1 brothers and g=b+3 sisters.

So g-(b-1) = (b+3)-(b-1) = b+3-b+1 = 4

Answer: 4

Edwin