You appear to have a lot of residual anger resulting from the hurt you have suffered, and are quite reasonably looking to share some of that out.
This is not the way forward for you - attractive and temporarily satisfying as it may be.
Your husband's ex-relationship's parents are not to blame for what has happened, and it is unfair to cause them upset for the actions of their daughter, although i can see from your perspective that you wish to cause her some direct upset to compensate for the pain she has caused you.
Your way foward is to work on your marriage and see if it can be re-built.
Re-establishing trust will take time and patience, and only time will tell if this will work or not - you say you are happy now, which is a god place to start.
By all means write your letter(s) - just don't pst them, keep them safe for referral when needed, and eventually, when the pain has receeded, you will be glad you acted with dignity, which is always the best way for you.