In the Season 2 finale of Supergirl, Rhea reveals that she used silver kryptonite to turn Superman to her side, making him think that Supergirl was General Zod.
Why didn't Rhea just use the silver kryptonite on Supergirl as well? It's not like silver kryptonite gets used up, and there was no sign that she lost it.
I mean, best case scenario, she gets two Kryptonians under her control. Worst case scenario, they fight amongst themselves just like they were going to do anyway, but with nobody holding back (and thus neither is interested in stopping their invasion).
Or heck, she is already willing to play dirty during their one on one combat for the planet. Why not just pull out silver kryptonite then?