I understand the statement that Jesus was the firstborn. Even if you only have one child the first is still the first. This was important because of Jewish law.
However, on the cross when Jesus gave Mary to the keeping of John that would have been a gross violation of Jewish law because she would have "belonged" to her "other children". So you (general you) mean to say that Jesus obeyed the law except for that one little bitty thing?
"Jesus was the firstborn"
"Firstborn" refers only to the male. The first son, even when preceded by four older sisters, is still the "firstborn"
Firstborn has nothing at all to due with the number of children.