NTA. Have a strategic illness tomorrow. Don’t ruin the shower. Tell Emily the next day. Maybe tell the MoH first if you like her? Emily is going to be dealing with a bunch of stuff, including her brother lying and letting her down so badly. And offer Emily your dress. Especially if there are an equal number of groomsmen and bridesmaids. That way she can find someone to take your place if she wants. (Yes, it may require an emergency alteration, and yes, I’ve known this to be done. Of course it’s easier when a size 16 dress is becoming size 6 than other way around. But that’s Emily’s concern.) I am sorry and it sounds like your ex isn’t handling this as well as you thought.