I am so sorry to hear of your difficulties. Only you know what will bring you peace. Nowhere in your situation I would probably want closure and would honestly share that I am sick of the threats and the emotional abuse and the drama and I would wish both your stepmom and your dad on their way with love. And each day pray for them but stay away. If you per dad wants to make amends to you after your stepmom dies let him put in the effort. Personally I have never liked the silent treatment and would prefer to close off and say thanks but no thanks to these people that you find damaging ungrateful and quite awful. To pray for the, that they both be blessed with their hearts' desires is also helpful whether on believes in God or not.
Good luck-if you so choose to re engage you probably need to consider doing it without an expectations...
Cath