Oh siblings- feeding question

[EmilyS]

I have recently started my first year in college; since I am no longer home often enough to feed my two corn snakes weekly, I put their feeding in the control of my younger brother.

Well for some reason he was unable to feed them today, so he handed the task over to my sister. Unfortunately, he forgot to show her how to feed them. I just got a call from my sister double checking that it was ok to feed them frozen frozen mice (no dethaw). I told her of course not, but it was too late as one of the two snakes had already started eating.

This is probably bad right? What will happen? What should I do?