Well, I have a 17 year old (kind of step-daughter), my boyfriend's daughter but we all live together. If it were her and I think her mom would agree with me, I/she would want to be there for her. You've said that she already knows you think you are so I would tell her and maybe she'll go with you too, might make it a little easier for you for her to be there no matter what the result you get. So, that's my opinion as a mom

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I'll add this now, because Amber and I just talked about this situation and she said the opposite that you shouldn't worry your mom about it until you know for sure.... I still say you should tell her! but that's only 1 mom's opinion. So, not that you asked for it but you have another 17 year old's perspective and a mom's perspective.