Varus's bow may not be a darkin
Sure rhaast has quotes to varus. Telling him "the 3 or 5 of us will lay waste to this realm." But to me it seems like rhaast is mistaking him for a darkin. If you go on the link you can see riot stating that darkin may not know of each other.
While they know how many there are, the darkin may not know for sure who exactly are the other darkins. Only aatrox and rhaast know each other. I also think that if rhaast were to discover varus he would mistake him for a darkin because of all the corruption varus has and a living bow. Rhaast calling him demon pants as well.
I think rhaast doesn't know if the bow is kin. On top of that rhaast does not know the name of the bow if it is even a darkin as he has always been saying varus's name. Maybe so riot doesn't give too many details away or riot is trying to play with us.
Also the bow doesn't even look like it matches the theme of a darkin. Since rhaast and aatrox are both red, grey dark color. The bow (or varus) is purple with black. Aatrox actually can make his blade purple so I guess there is some resemblance, but there is no red or grey.
But who know. I guess we just to wait for Aatrox's update which I think they will give more lore.