I saw the mods are considering locking this thread, so this is only a part of my response, addressing the lead up to OP's son being arrested. I'll edit in more as I go. Plus formatting apparently.
My wife and I reported him to the authorities. My son was arrested in December and held in prison for a several weeks because I refused to provide him bail money.
My wife slapped her across her face and sent her away. I don't think either of us will ever forgive her. And I hope she will live with this guilt for the rest of her life.
You and your missus have thrown enough gasoline on this bonfire already, with amazing results. Maybe you should stop making decisions for a few minutes, and butt out of everyone else's space while you contemplate your responses throughout this:
Response one: confront son.
Wtf did you hope to achieve by pitting these two into a he-said-she-said situation. At face value this means you didn't want to believe her. If she really was a victim, congratulations on the way you stood up for her..
Response two: you sat on your ass for a few days.
Wait, wat? WAT? A kid you ostensibly care about told you she has been sexually assaulted, and you hem and haw for a few days before deciding what to do? With her presumed abuser still in the house.
Idle speculation: if that's the kind of parental support your step-daughter grew up with, it is not surprising she didn't realise the the severity of her false accusation. You didn't bundle her off to a doctor or specialist for care, you asked your son if it was true instead of believing her. You didn't remove her alleged abuser from her home, you sat on your arse and did nothing supportive for a few days. Somehow this kid grew up to believe that sexual abuse was a kinda serious thing, but not really serious... enough to get you out of trouble but not enough to get someone else into trouble. I gotta say, your initial response backs up that worldview.
Response Three: suddenly escalate to 'the authorities'
This is shamefully telling for you. Somewhere in the following days you did realise this can't be ignored and you must be seen to act. 'Reporting to the authorities' sounds firm, hard line and proactive - yeah, you were strong and took a stand... with the bonus that hand-balled your problem to someone else.
I'm not saying you should have kept this an inhouse family matter - I'm just observing that by this point of your TIFU you still haven't sought care for your step-daughter or protected her from her abuser. You went from inaction to some-one-else's-job.
Idle speculation 2: if you had taken your step-daughter seriously and immediately removed her 'abuser' from the house (didn't need to be harsh about it), and gotten her expert care and treatment, then (i) she would know you take her seriously, (ii) she would have a chance to realise what she said is not a trifling matter, (iii) she might have had an opportunity to recant her accusation while being treated by a professional trained in mandatory reporting. Just a thought.
Response Four: no bailout for son.
A solid zero-tolerance response on your part. You're telling the world that blood is not thicker than water and you won't stand stand by him in this scandalous situation. It's a good decision, proving you will put your step-daughter's well being first... except... you didn't put her first when this all started. A