I'm sorry this has happened, but I can't help thinking that you, dear OP, want to remain in a tortured state of not-quite-knowing what went on. I suspect that the reason you couldn't "find the heart" to set up a camera or speak up sooner than you did, is because YOU really don't want to know the truth.
In the short run, your willingness to NOT gather evidence of what happened between them allows you to * try * and stay in your marriage. That's a valid choice, but one that leads to immense long-term paranoia and resentment. It also usually leads to a long, bitter marriage that ends up falling apart.
On the other hand, if you'd held your tongue and installed hidden cameras, you would have had a shot at better understanding the nature of your husband and sister. The worst that would have happened is you'd feel guilty for catching nothing and installing cams, and could work at your trust issues in therapy.
Since you don't seem to have enough respect for yourself to honestly deal w/the likely reality of your husband's infidelity, I will make a recommendation for someone else who might be in a situation similar to yours and might be reading this thread: Have lunch with your sister. Lie. Tell her you know all about her and your husband, so she can stop pretending now. Just say that with a straight face and a normal voice. No anger. Say you have video footage of what happened and if she still wants to salvage a sisterly relationship with you, she will apologize to you for what she did. (That in no way truly binds you to a r-ship with her though. Your real mission is lying to get her to tell the truth.)