I think you're a sweet friend, Amelie, for caring this much, and that this issue is the INFP's fault. Sometimes our Fi gets us selfish, and in less mature people's cases, it gets this "I am so upset I don't want to let this go" mentality. She must know she's being unreasonable, inside. Because she is. You're being a good friend to her, though; it's up to her to see that and be just as good a friend to you and her bridesmaids. Till then, just be patient and do what you've been doing.
Best of luck and hugs.