My husband kept a huge secret from me

It is understandable that you are questioning whether to trust your husband as you learned that he was involved in an incestuous relationship with his sister ,and you believe he hid the information from you for years. I am sensing that you believe he may still be involved with her as they work together and see each other daily.In our society, incest is considered a taboo, and this might clearly have been the reason he could not share it with you. In addition, it is not clear whether the incest emerged as part of experimentation,or whether it was a traumatic experience for both siblings in which one of them played the role of the malevolent perpetrator and the other the victim. In any case, there is complexity around the dynamics  incest and it is often necessary for the individuals that participated in it to  undergo a healing process. In like manner, it must have been distressing for you to uncover this event in your husband's life. You may need to explore these issues in couples counseling with a professional who has training and experience in the family dynamics around incest that can shed further light for you. I hope that through that process, you may be able to figure out what direction to take, in terms of whether to preserve your marriage or not. 

Trust is a necessary factor for an intimate relationship to feel safe.

The person who must repair the trust is the one who has broken this trust.

The best way to find out if your husband wants to restore your trust in him, is to ask him this question.  He may not be willing to meet your request.

If he hesitates or becomes defensive about the need to restore your trust, then ask him to think his decision through and tell him the serious consequence you are considering.

Ask him to reflect for a few days or a week and then to address the topic again.

If you and he consistently find that he is unwilling to do anything at all to regain your trust, then I agree you have serious thinking to do as to whether or not you'd be able to live within a partnership based upon trust, in which you don't trust your partner.