I'm going to say I think she was entirely serious in her threat, but on Page 26-27 you can see the Fire of the match Flare Up Wildly and then she Covers her Eyes. Given how Magic has behaved around MC so far, I'm guessing she was casting the spell, then when it flared up she realized that if she did put him to sleep for a few years, that the magic would go wild and cast the spell on Herself as well, putting both of them to sleep.
So she abandoned that plan and shifted tactics to put herself into a better position to figure out just what the hell is going on with this dude.