D.K.
She has raging hormones, moms already that have kids should totally understand that. I can say that when you have kids, you get it, until you do, you don't get how excited kids can be.
She has the stress of being pregnant right now, with her first, then to be engaged too. That is a lot at this time of year. I am sure when I was pregnant a room full of crazy kids would have driven me up a wall too. I would explain to family members is it hormonal more then anything, I would NOT tell her to lighten up as I am sure that is the last thing she needs to hear. She will understand as her child next year will be a crazy one in the mix and she will then understand.
Don't worry about come backs, what she needs is support and reassurance. Being a first time mom is very scary for anyone and if you just keep telling her she will be a great mom and how fun it will be her child will be in all the chaos next year she will maybe feel more at ease.
Just be patient with her. Seriously did you want your hubby to tell you to lighten up with your first time mommy jitters?
She just needs her family to be understanding right now.