As a point of clarification she uses it in this sense:

"Supposedly, Aunt Sue is coming down this weekend"

What she means is "My mom said, "Aunt Sue is coming down this weekend"

What I interpret this to mean is "Aunt Sue says she is coming down this weekend. But, She's so flakey we shouldn't plan anything around her"

The but could really be anything, and it all depends on context, shared knowledge of the situation or person we're talking about.