Taking a stand to be able to get sleep was more about fear. The best analogy I can use is how a person reacts to being dunked under the water and held there. Eventually you will come up fighting... .perhaps you will exclaim a few things.
I think you have hit on something really important here--you have some very real fear around your sleep problems, and people threatening your ability to sleep peacefully.
Set aside your logical/rational side for a moment, and don't think about the reasons that is important. (Put it in a box somewhere safe; it is important and you will want it later

)
Your fear, no matter how justified it is, is still fear that you have to deal with when you choose how to act. And if the fear hides behind ANGER, which you've also experienced, you have to deal with that anger as well. In other words, this stuff is pretty triggering to you.
Acknowledge those feelings, be ready to give yourself a time out to deal with them before you engage with your wife or her FOO on these issues. You will be able to handle the situation better that way.
(OK, you can pick up all the rational stuff about your medical sleep issues again)
There is a common misconception with my wife and her family that "I sleep just fine, I just have to have a machine" Then stories are told about others in the family that have machines and "sleep anywhere" and "I could be like them if I wanted to" as "proof" that "I'm just being an a$$".
Whups, too soon... .don't use those rational thoughts to try to bludgeon your wife or her FOO into agreeing with you... .put them back in the box, and get out your tools for dealing with pwBPD and other mentally ill folks!
This is a boundary enforcement issue, and there is only one thing that matters. It doesn't matter what they say about you. It doesn't matter what they think about you. If you have to "just be an a$$" to get your sleep, go ahead and do it. But be an assertive one enforcing a boundary, instead of a raging one.
They don't need to know why. They don't need to agree. They can say snarky stuff about you. All they need to know is this.
YOU ARE NOT GOING TO COMPROMISE ON YOUR SLEEP NEEDS. Keep your eye on the ball.