They add to the sylphs, if you hold down yourn thumb on the name while playing it gives a brief description of the word and it's effects
The problem i have is that the description doesn't really tell me anything and I don't see any visual difference when I have it, and then how does it actually work? because it doesn't reduce the zephyrants while also increasing sylphs which is what I would assume "convert the sinuates" would mean originally, because it doesn't decrease it, it means the numbers *should* interact in some way, but how? is it a "you get sylphs based on the lowest number between the amount of sinuates and the amount of zephyrants" is it some other form of multiplication?
From what I can tell, sinuates give -3 sylph per stack, each upgrade from sinuates multiplies your sylph per click by that many times +1, and zephyrants are just a place you can throw your sinuates at to decrease how much theyre removing from your sylphs per tick, not actually converting them into sylphs.
I went and started a new game just to test it, and I didn't get celestines because I didnt want too much numerical clutter, from what I can tell having zephyrants makes no immediate difference aside from decreasing your sinuates once and the number of zephyrants does nothing at best maybe it decreases the amount drained by that flat amount, but thats not what the description reads.
I don't mind in game descriptions being vague I just wish I had a place to go to for actual numbers.