Castor Oil? That's a first. I can see it helping hair from breaking (like lashes) but brows don't really break. I tried this experiment on my eyebrow clients in New York, and got great results. They were older with eyebrows that had stopped growing in the outer half of the eyebrow. I had them scrub their eyebrows (with a facial scrub or sugar, it must have some texture to it, or a toothbrush) 2/3 times a week. This gets rid of dead skin, and stimulates the hair follicule. After 1 month, they all came back with eyebrows that had started to grow in. Mind you, this was eyebrows that had stopped growing years ago, so I'm sure this would work on anyone with thin or growing in brows. If I didn't see it with my own eyes, I wouldn't have believed it. Try it and let me know what happens! I'd love to hear more feedback on this.
Also, those brow serums? Have yet to see ANY results from any of them.... they're like lip plumpers. Great in theory but doesn't really work.