I 100% agree with living
This toxic family dynamic with narc MIL, interloper, OH, DGC and FIL is a well known and documented dysfunctional family system. You can research it all online.
There have been many, many threads on here with the same set up and the issues, behaviours and next actions of each of the characters / roles in this are so classic and predictable that once you know what you are looking at you can step back and see the whole pantomime play out.
Once you get educated on dysfunctional narc families you will feel less isolated and misunderstood - because currently you are the only one of the 4 that can see it. The other 3 are so bound up in emotional FOG it is their normal. However if you look outside of this family for information and guidance you will be supported by many, many people who are in the same situation.
And once you educate yourself it will relieve you of the massive burden that you are the unreasonable, crazy one.
This clarity will inform you and energise you that this is all wrong and you need to continue to protect your child, your bonding with your child and your own MH from the toxic antics of your personality disordered MIL, even more. You feelings, thoughts and actions are not wrong - they may feel extreme because you are in an extreme situation. This set up is not normal and normal rules don’t apply.
The challenge is whether your OH comes to see it in time and support you. That’s his journey to make, or stay in denial and leave you to protect you child alone.
Leaving him isn’t the right decision for you now. But knowing your feelings are right and not being subjugated by MIL is important. You have the power here.
You don’t have to do it aggressively.
You can express your boundaries calmly and with dignity and be clear and carry out the consequences. The consequences are instant and consistent ACTIONS not engaging in words and rows with a loon. It’s stating ONCE that your wishes have been broken, that this is your DC and you will now leave, ask her to leave, not open the door.
The same with your OH.
I would consider taking some emotional distance right now to settle, reset and regroup - so tell your OH that you won’t be seeing, contacting or discussing ILs for the next 6 weeks.
Get informed. Know you are right and they are wrong. Respect and protect your own feelings. Stay strong and calm. Have tactics and coping strategies in place - hope that OH becomes enlightened and eventually works with you to manage this. But even with him - don’t waste your breath - signpost him to some resources, he is the one who needs counselling - show him with actions not words. The sky will not fall in if his mother hears the word “No” - she will run off crying so what - weather it. She doesn’t get to dictate your life and rob you of joy in these finite precious days of motherhood with her threats of tears and trampling your wishes.