of you had any kids, and predicted this order exactly, it is about a .007% likely to happen. However, what I really think you are asking is after the first sibling had the gbbgb pattern, what is the likelihood of the others matching that.

If we assuming the "51% vs 49%" boy versus girl chance is correct and no other item factors in (such as genetic bias for your particular family), it looks like a .21% chance of happening.

That is one in 500