With respect, you don't understand fully as ESTA is not a visa and your OP stated 3 months, not 90 days, which can be the difference between overstaying or not overstaying. Also you say you are not worried, then why has she not applied for an ESTA yet if it's a quick online process?
You say she is studying - where is this 3 month gap? 3 months also seems a very long time for a 'reward'.
It's good she won't be travelling alone and you will be there with her. I would expect you will need some sort of proof that you are related, and an AoS from you and your husband stating you are covering all expenses, in addition to the parents consent.