I'm pretty sure that the stats show that while having older brothers makes a man more likely to be gay they opposite is true of women, the first born is most likely to be lesbian or trans.

With men it's partly due to mother's getting an auto-immune reaction from bearing sons so they are less likely to product enough testosterone in later pregnancies. Maybe they produce too much for the first and turn their daughters gay.

The oldest daughter often acts as a third parent maybe it's good genetics wise if they don't have kids themselves.

Certainly the oldest daughter is more likely to be instilled with more masculine and self-reliant virtues by their parents but that having an effect on sexuality would go against the born this way narrative. They are more likely to have poor parenting as well because the parents are just learning what to do with the first born.

There is also a weird trend that the more younger sisters a woman has the more likely she is to be overweight. That would also seem to suggest that older sisters do sacrifice their own fertility to support their younger siblings.