Gender reveal issue

So, I am in charge of my sister's gender reveal.. I went to the ultrasound with her and got the envelope and I get to put it together for Friday's big reveal. She has had fertility problems over the past few years with 3 miscarriages during 1st trimester, one that happened in february. Her and and her husband are over the moon about there new baby and making it this far... But there's a problem. They want a boy. My family is convinced she is having a boy. I am pregnant too (but due later) and she has told me so many times how upset she would be if she had a girl and I had a boy because I have two sons already. She has done all the internet quizzes and ultrasound ramsy tests(early on) and heart beat analysis, and she is convinced it is a boy! She even refers to her baby as a him constantly...But... It is a girl according to my envelope and ultrasounds, believe me, I checked 5 times. I am so nervous putting all these decorations together... I don't want to be the messenger in this situation... I don't want her to be upset at this big party.. Has anyone had news of a gender they weren't thrilled about?? Obviously she will love her baby girl. Especially for her being a rainbow baby. I know that. She will be happy, but maybe not right away... Pregnancy hormones may make this emotional and I don't know how to make it easier..