Am I being petty?

So this weekend we announced our pregnancy to husbands family at a big family meal. Everyone started screaming and crying and was so happy, it was great, and then my sister in law and her husband blurted out - we are pregnant too!
She is only 5 weeks and literally just found out. Am I right being peeved that they had to announce at exactly same time? I don't know why they didn't wait for a few more weeks as they've kinda ruined the surprise of their announcement too...
It's really been bugging me and I'm trying my hardest to get over it, perhaps I'm being over sensitive?
And for the rest of the night my SIL was all sulking and upset and everyone kept going over to her and fussing all night. When I asked her if she was ok she said she was fine.
😕