Family member throwing a fit

Has anyone else had a family member on their S/O's side of the family that has thrown a fit when they found out there's going to be a baby?  My boyfriends sister just found out because she snooped in her moms phone through a private conversation between his mom and me.  She is 29 years old and is now saying she isn't coming to Christmas family dinner at our house (she also lives in the same house) and how this is such a negative thing because we aren't married.  His siblings and mine were going to find out tonight as their Christmas present because we are trying to make this into a positive. How on earth are we supposed to handle this?  Everyone is just babying her and her reaction and she is staying with her parents tonight instead.