A month for us is like a year for them. To her this one thing keeps being brought up, and brought up, and it was “SO LONG AGO”. It sounds like (to her) it was a white lie to get out of an uncomfortable (to her) situation. The real issue is she felt she had to lie concerning her dad, and you are easy to throw under the bus. Now she sees you are not so easy and is lashing out. Why is everyone walking on eggshells concerning the father? Sounds very reactionary.

For you this is one thing. Her parents (mom and stepdad) see this every day. There is probably similar things that go on every day.

Leave the girl alone. Talk to the mom. See the family a year from now when everyone’s more mature. Don’t get cast into the role of the Least Favorite Aunt. It sounds like she’s acting like a terribly typical teenager.