Welcome to the forums. I am a newbie here myself but I must say that I have gone through such situations several times and truly appreciate that they are sensitive, tricky and consume a lot of one’s mental space. Well done on identifying the issue and trying to solve it.
Easier said than done..but this is what helped me.
Over the years, I have tried to be more interactive and open with my in-laws. Very difficult for an introvert like me. Sometimes it has helped, sometimes it hasn’t but am learning each day and adamant that things will change for the better.
I have come to realise that usually no one is nasty just for the sake of it. There’s a story behind people’s different perspectives and why they act the way they do..and this is what I seek to identify..eg in this case, why do you think they are overstepping the boundaries? Keep making the change you would want to see, although slowly, taking baby steps..so eventually it becomes part of the new normal. Be consistent with your approach.’Sustain the change and allow them to get adjusted to the new norm.
When your partner is ready to listen, do share your thoughts in a calm way.
Hopefully things improve for you quickly.