Hera (as far as we know) has no siblings. We don't know that much about her childhood, but my headcanon is that given that she was growing up among her father's resistance to the Separatist occupation and later his Free Ryloth Movement under the Empire, she spent most of her time around other resistance fighters. While this sometimes included those fighters' families, not that many also had children, so Hera was more accustomed to being in a community of adults than with very many others her own age, and into adulthood she had usually found company among others older than she was. Probably the first person close to her age with whom she had that kind of peer relationship was Kanan.

Kanan also didn't have any siblings, but until his teens he grew up in the Jedi Temple with plenty of other children (as well as a community of adult mentors). He had some close friends among the other children during his training, though he also became more distant with them once he was chosen to be a padawan, both because it meant he was leaving the Temple to join fight in the Clone Wars with his master, and because his friends became jealous of him for having been chosen to advance before them. And then Order 66 happened and all his peers were killed, and he didn't become too close to anyone again regardless of age, until he met Hera.