I guess you will really need your husband to step in to voice out. You're correct to assume if you personally voice out or worse, lash out at them, they will think you're disrespectful. But also, if they have any common sense, they should know how not to push the wrong buttons of a pregnant woman.

But I guess it also falls back to how your MIL was treated when she was pregnant with your husband. If her experience was such, then it would be likely she will follow suit. I guess I am the lucky one. My mother didn't like how my grandmother nagged her, so she decided to break and end the "chain/circle" and not pressure/nag my wife at all. If anything, my mother will text me first to gauge my understanding of my wife before suggesting/advising on anything to her directly.