That sounds incredibly challenging. I don't think you are a "bad" mom by any means. You sound like a very loving, very concerned mom, doing the best you can with a difficult situation.
I really think your family needs professional help to deal with this.
In the meantime, I think the best you can do is keep trying to protect your son, while also recognizing that your DD cannot voluntarily change her negative obsession with him, and that any kind of punishment or firm words are contributing to her self-hatred.
Maybe set aside some one-on-one time to reconnect with each of them. Doing something that they enjoy, with little to no correction, just being connected and loving, will go a long ways to helping offset the hurt and pain each must be feeling.
I really think your family needs professional help to deal with this.
In the meantime, I think the best you can do is keep trying to protect your son, while also recognizing that your DD cannot voluntarily change her negative obsession with him, and that any kind of punishment or firm words are contributing to her self-hatred.
Maybe set aside some one-on-one time to reconnect with each of them. Doing something that they enjoy, with little to no correction, just being connected and loving, will go a long ways to helping offset the hurt and pain each must be feeling.