Killing yourself is that easy, if you know how to do it, just eating your own tongue can make you die that easely, but yea it's should have take a least ten second of suffocate before dying if she did that.
The only way to die so quickly in this context was to break a poison hidden in your teeth, but i doubt she have that, i mean, it's just a weak sister.
So yea it's a little plot hole, but really little, she just eat his own town and die quickly than it's should been, it's not really a bad plot hole, you can just ignore it, if she was dying in suffocating, it's will change nothing in the end.