5 L&R,
I worked some time ago with a female radio tech and recall having a conversation with her on this very subject, progeny not sibblings.
All of her colleagues, as best I can remember, had female off- spring. The reason was put down to so much time spent in a radar environment. I'm not too sure if it was Wx radar, terminal radar, or both, that is thought to render the male sperm less successful in the quest for fertilisation.