Don't write a letter to your aunts and grandmother, call them.
Explain in real time/in person on the phone what is going on, what has gone on and the situation. Explain to them that you love them and cherish your relationship with each of them and under no circumstances do you want that to change but you just cannot have the relationship with your parents. Welcome and answer their questions.
Then cut your parents out. DNA is not a reason to have relationships with toxic people.