I'm going to spill the secret that my sister's husband was previously married
That is very shocking. Why have you decided to reveal his secret?
She doesn't need the nerd since he is already married.
Interfering negatively in a marriage that is not your own is wrong. You don't gain anything from doing this and you may end up feeling very guilty for it later in life.
I don't see why it matters? They are just going to get divorced eventually.
Even so, you may end up causing unhappiness with the marriage even if you will tell the truth. I would just recommend you tell the husband first to reveal it himself to your sister, rather than going against her husband's back. That way you will not meddle with their marriage and the husband will have the chance to be honest.