It’s not secrecy or lack of clarity my friend, it’s called betrayal.
Cheating + gaslighting + blame shifting = Infidelity
Always trust your gut it rarely is wrong.
You feel like that because nobody who you cited there in your story has a sliver of respect for you as a person.
They treat you like a doormat and seems they are confident that you’re not going to show consequences, you will just get sad and suffer in silence.
Do I get it right?
Because brother, I don’t think you are wrong here.
And I ask you, are you ready to begging brewing inside you the biggest "fuck you" to everyone who gaslights you and reclaim your self respect and agency?
This is the most important thing, about the story you already have all the answers, and yes, the answer is yes, she cheated, to what extent is irrelevant the point is she has no quarrel into abusing you to the point of betrayal trauma, and she (and her sister, and everyone else you cited) seems to believe you are so weak, so accommodating that you would never react accordingly, so they do not even feel the basic shame.
They have no respect for you, is not even roommate status, that’s furniture status.
Seriously, I hope this makes you angry enough to start brewing a huge F bomb.
If this resonates then we can talk how to channel it properly, because freaking out won’t do it, there is way to put this kind of people in their place, and that starts with your healing.
That’s the first I put, talk here, you need clarity and people here can hear and understand you.
When you have clarity you will get back your agency and demand the respect that is due to any human being. Boundaries man.
And your wife and bratty sister will be crawling into a corner, I guarantee you.
You have been heard.