Hello Michelle! Your letter might make you feel good for the moment, but you won't secure you the closure you desire, it's not good reliving the hateful acts inflicted by your family who are blind to emotion. I know letters don't work, you will fuel more bitterness in your own mind, your parents don't care or they would have shown you love.
In a perfect world, where people actually cared for one another, worried about bring other pain, your letters might touch their heart strings, if they were human! It won't with this religion, the most nasty people are the most self-assured of their standing with God. You can write a letter to yourself, address how you want your children to have better parents, scrible your desire to break your family cycle, write how you want your kids to have the best you can give.
Your time would be wisely invested by focusing your energies on your family that cares about you, you have to break the child notion, that outside family are worth chasing, instead of the waste of emotion they truly are! A life well lived is shown by the children we raised, and how well they fly through life, each obstacle thrown their way!