Infertility is not hereditary. Obviously if someone was infertile, there would be no offspring to pass on their defunct genes to! However a low-sperm count could be genetic.
There is also some medications that women have innocently taken in past that have caused their offspring to have difficulties - such has an anti-morning sickness drug from the 60's caused many girls to be born with an incomplete cervix.
There is no medical reason I can think of that would cause both male and female offspring to be infertile. It could be just a coincidence his siblings are having difficulties conceiving. And presumably your b'f is quite a different age from his siblings and from a different relationship, this means that their physical environment was different growing up. This would rule out an environmental problem (ie exposure to radiation).
I would not worry too much about it, but if you are trying to conceive and are having difficulties, it would be a valid point to ponder.
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