Thinking about this question again. There are often a couple of spontaneous mutations in an individual's cells, so if one did complete DNA sequencing, one may in fact, find subtle difference between the DNA of identical twins.

It is always interesting when a family has multiple same-sex children.

However, for two children,

1/4 of the families will have 2 girls, 1/4 will have 2 boys (half having same sex offspring), and half having mixed.

for three children,

1/8 will have 3 girls, 1/8 will have 3 boys (1/4 total), and the rest will have some combination of boys and girls.

for four children,

1/16 will have 4 girls, 1/16 will have 4 boys (1/8 total), and the rest will have a combination.

I.E. It is not too uncommon to have several children of the same sex.

In your case, should you and your wife choose to have a second child, attribute a 50/50 chance to male/female, and don't be surprised with either outcome.

It would be odd to have two siblings, one with four daughters, and another with four sons, but such an occurrence isn't impossible.

Your brother already having four daughters doesn't change your chances of having 3 more sons.