What a hard, hard situation. I’m glad you wrote.
First things first: Please remember that partner love is different than parent-child love. Whatever your parents’ problems, I doubt very much your mom wants to “divorce” you girls too. It’s only natural you would worry about this. Most kids do.
As for what to tell your mom: I encourage you to have a heart-to-heart with your mom about how you are feeling about the breakup of your family and her involvement with the new guy. How you say it will likely decide how she will respond. If you come at her with anger and accusations, she will be defensive. If instead, you let her know how much the situation pains you, how worried you are about her and your dad and how confusing it is for you to figure out how to relate to the man in her life, she will probably be more willing to really talk with you.
I found it curious that you never mentioned your dad in your long letter. He’s your parent too and has some say and what happens with you. Regardless of what goes on between him and your mom, he can still be your father. Right now he has lost his wife and his girls. I have to wonder how he is dealing with that.
Ideally, both of your parents would be talking to both of you girls about how parenting is going to happen now and what they expect of you if they find new partners. Since you are teens, your thoughts and feelings should be taken into account. You might want to suggest to your folks that some family counseling is a good idea. It might not save their marriage but it might help everyone figure out how to be this new kind of family.
I wish you well.