Ticklishness may be heriditary, but not necessarially. I'm very ticklish, but I know nothing about my parents ticklishness, nor do I care to know. I'm also an only child, so I have no siblings to know about.
The only information I have on this topic is about my best friend's family. His sister, who is now in her 40s, has supposedly never been that ticklish, even when she was younger. The only way his sister would laugh if tickled, is if someone got her if she wasn't looking. Other than that, nothing.
His mother, on the other hand, is supposedly incredibly ticklish, and always has been, even though she's in her 60s. A couple of years ago, my friend and his mother took a yoga class together, and he was telling me an experience where the yoga instructor did a demonstration on the mother's foot, and the mother went ballistic laughing, even though the instructor wasnt trying to tickle her.
My thoughts on the subject, ticklishness may be heriditary, but I dont think it necessarially is. It depends on the family, and the person.
Mitch