Yes you can candle them. I wouldn't wait until day 26 in case they are rotten and may explode, but should be fine to wait until tonight. Your broody won't be as upset that you are removing her eggs since she can't see well in the dark, and it will do no harm to any viable chicks when you quickly remove and candle them, then replace any viable eggs. Day 18 is very early for a chick to hatch under a broody, but I've had some hatch on day 19, and many that hatched on day 20. It's not promising that none have at least pipped on day 21, but waiting until tonight will give more time for any viable chicks to do so.