Sidney could cleary show regina anyone, anywhere, regardless of whether or not there was a mirror or reflective surface in the area. Remember, didn’t she poof Charming into her dungeon while he was running throught the forest as she watched him through her mirror. And she spied on Snow talking to the dwarves (“She’s hanging out with Dwarves now?…When did that happen?” LOL). So this requirement that there needs to be a reflective surface for one to see another seems to be new.
Well, this requirement has been around since S1 (since Rumpel covered up his mirrors), so this is something that has always bothered me. I just hand wave it by saying she put invisible mirrors in the EF.