nervous, fearful
伈 centers on an internal state of unease: being nervous or fearful, with the heart/mind unsettled.
伈 combines 亻 (person) with 心 (heart/mind), suggesting a person whose heart is unsettled. The exact historical development is uncertain.
A person 亻 with a heart 心 that is trembling: imagine someone whose heart is visibly nervous, making them fearful.
For シン, picture a person whose heart is so nervous they can only think of the word 'shin' (as in shin splints, a painful, fearful feeling). The sound 'shin' cues シン while the person trembles.