Sorry to hear that you have had to endure that for five years.
Keep the screenshots somewhere safe in case you need them but maybe out of sight. I did the same thing. Every single word I started to document it because the gas-lighting, blame-shifting, and projections etc I wanted to assure myself it was real.
The only good news about being broken down to absolutely nothing by your experience is that unlike the NPD person, you can rebuild yourself. There was a point of course where nothing about being yourself anymore is joyful; none of the things you once enjoyed mean anything to you. I promise you that if you were strong enough to endure narcissistic abuse, you are strong enough to bring yourself back and a better version of yourself at that. It is like depression you have to take it one day at a time with goals, mindfulness, and digging yourself out and time heals a lot of it. The emotional numbness and despair do lift.
It is far better to be lonely and depressed than it is to be in the company of a narcissist. There are billions of people in the world and you can find others and this is something that might push you to do so while you are working on yourself as a way to fight back. I say this as someone who lives in an isolated area and is mostly alone so can relate how it can seem like life or death socially but the quicker you can get away from them the better off you will be mentally, emotionally, health wise etc.
We have the ability to change our perception to our external world, and if he is making sure he is your only pain and joy, then you need to make sure how you decide to respond to it.
There is a great quote by Marcus Aurelius that is eye-opening:
“If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment.”
That is a very difficult situation with your sister’s fiancé. It might be best to try and let that go as an injustice. If he’s not willing to stand up for you then it’s possible he doesn’t want to go against the herd and have the NPD wrath against him and you can be thankful in a way that you are not such a coward, and if it was you I am sure you would help.
“Lost, broken, confused, and dead inside” it’s a nasty little bit of irony about narcissistic abuse, that is how a narcissist describes their inner landscape. It is transference – they steal everything positive about your personality, soul, and life and in exchange leave you with their emptiness. Take it back, cut them off, and never let them in again or this will always be how it is. They are always “the victim,” it what they “are,” it’s what they do; it’s how they hook people etc.
I won’t confirm that you are a victim although I am sure you have been victimized. You are still here so you are a survivor!