Thumb sucking blister on lip

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My 22 month old has been sucking her thumb a lot this week- she has a new baby brother, and she’s teething with her second molars. There appears to be a blister on her lip today, and I think it has to be from friction. She hasn’t been around anyone with a cold sore, so I don’t think it’s that.

How do I treat that? When I get cold sores, I use alcohol on it, but not sure if it would be useful here.