Aw, I'm sorry.
Did SO tell his parents not to tell anyone?
We had to explicitly spell it out to DH's parents, and they still spent ages asking if they could just tell X, or saying that they'd be telling Y if we hadn't announced by then, etc... They very much see it as "their" news. I disagree with their view completely - they'll get plenty of chance to tell their friends that they'll be grandparents after we've announced! - but perhaps your in-laws have the same view...
I know my DH felt a bit stuck between a rock and a hard place when they reacted like that. He'd been so excited to tell them, and it hadn't occurred to him that they'd then want to take over spreading the news. To be fair to him, he was clear with them that they weren't allowed to tell anyone yet, and they haven't, but he did feel a bit stuck that it caused a lot of anxiety despite that.
Overall, you're not being unreasonable to be upset - but if this has taught me anything, it's that it's really better not to tell some people until you're ready for it to be common knowledge, and sadly that includes my in-laws. If we have more children, they won't be being told until we make a general announcement, to avoid this in the future!
All the best with your pregnancy 