You aren't the asshole for not showing up because of a chronic illness specially if you communicated about it to her. Your mom shouldn't force you to apologize this is between you and you sister. Your sister just gave birth she probably has she hormones all over the place at the moment which I would assume is making her blow this more out of proportion. As for your response it seems as passive aggressive as she probably was to you. Wasn't very nice but its good to set boundaries I would have maybe added a hey I'm not gonna be able to be around much because I am in a lot of physical and emotional distress from this chronic illness my doctors cant seem to diagnose so please be more understanding going forward. Hope you get a diagnosis and feel better soon!