Reading everyone's posts makes me really have to commend this show. I love how these character we see for only 45 minutes a week really impact how we view our relationships (mothers/daughters, sibling/sibling).
Between Kate and Rebecca's relationship, I almost see a transition of blame during the three time periods we normally see the family in. When Kate was younger, it is readily apparent that she had self-esteem issues. Some of them seem self-inflicted like seeing the sweater sizes between her and her mother being different. That was totally Kate's perception. On the other hand, there was an episode were Rebecca innocuously dismissed Kate having a sugary cereal and told her to eat a grapefruit instead. While this advice was understandable and done coming from a place of love, it was insensitive to say it in front of her brothers who got to eat the cereal. It isn't the grand monologues that this show would sometimes have us believe happen naturally (I'm looking at you, Toby for the "Team Kate" speech) that impact our character, rather the little comments and gestures over a lifetime.
That being said, adult Kate was 97% in the wrong tonight. When you grow up, you have to recognize whether or not is the right time to take things personally. The best response for her mother's unexpected arrival would have been to politely thank her and keep it moving. Yeah, that "existing" thing? Absolutely horrible, for sure but I really hated how Kate told her that she "made her do it". No. Just no. I mean, for God's sake, at least try to treat her like a sentient human being, if you can't acknowledge her as a parent.
TL; DR:
Boo, Kate!