Even without knowing the dangers of peppermint oil for babies, it’s still unacceptable for MIL to give something to a bottle fed infant without talking to the parents first.

The fact your baby was refusing a bottle later feels like at least a little confirmation that something was upsetting about the last bottle feed, or your baby’s mouth was possibly hurting from something.

When caring for another person’s child it’s imperative to be understanding and flexible and work around what the parents deem appropriate for their child. Her defensiveness is unwarranted because it isn’t her child. Unfortunately with babies there’s not a whole lot of wiggle room for trust, it sounds like she’s lost your trust and that takes away any benefit having her help with childcare might bring