Lisp and Forth are both very simple systems in terms of basic concepts, and push them to their limits.

Both are quite flexible.

Forth often runs on the bare metal, and exposes everything below. Lispers are customarily more shielded.

Both encourage code to be treated as data and vice versa (without even looking at macros), but Lisp builds on the lambda calculus to do that, while Forth just sees code and data as being contents of memory.

In Forth if there's a bunch of data flowing into a word, you might put the number of arguments on the top of the stack and the other arguments below. Lispers would run away screaming from this manual and error-prone approach.

Lispers like to build e.g. their their own object system, if necessary. And in Forth it's easy to tag on a garbage collector---from within the language.

I hope that's some meat to back up my assertion. I hope I got some good examples. What I really want to say, is that the languages feel related in spirit. But less like brothers, more like to generals from opposing armies still respecting each other.