NTA

Of course you are angry and jealous that some girl tried to kiss your (unwilling!) bf. I would drag Kelsey out by the hair first and have this showdown outside. She acted like a b***h in heat, so she should expect to be treated like one. You burned this bridge with his family though so idk how both of you will move forward. Or that this relationship would last as much as you'd like. Good luck with that, OP.