I'm not sure what is going on, but it definitely sounds like something is going on. Maybe she feels that her family is not "perfect" and you might be coming across as judging of her and hers (you might not be, but if she's feeling vulnerable, it might seem so to her; or you might in fact be coming across that way regardless).
I think the best thing to do for your relationship if you want to try to help and also rekindle the friendship, is to ask her to go out to lunch with you (just you and just her). Tell her that you have been noticing what looks like a distancing of your relationship, and you're just asking because you're concerned (ie, is everything okay with her, or with her kids). Let her know you care, and then just listen. Don't judge or react, even if she says "you're always comparing and I feel like you think our family is not as good as yours" or whatever).
If she doesn't say anything, then be honest and open--let her know that you sharing your families accomplishments is not meant to be comparison or to make her/them feel bad. (in this scenario, you might mention the report card incident--but don't be accusatory or have a judging tone of voice). Let her know that if you've done something that has hurt her or her family, you want to try to make it right.
And then let her know that her niece really wants all of her loved ones to share in her special moment later this month, and that you felt somewhat hurt on her behalf that even though they knew about it well in advance, they made plans to do something else with another family.