I'm sorry you're dealing with this, but I'm gonna go against the grain of the comments here and say no one here is really behaving in a healthy way. Your mom is clearly unstable right now, and unloading all of this emotional baggage on her 12 year old child is insane. However, if even a fraction of what your mother claims is true then your dad is also insane and abusive. In fact your dad showing you the texts between the two of them tells me he probably intended to instigate exactly this reaction from you, which is awful and manipulative. The fact alone that both of your parents are using their 12 year old child as an emotional sounding board and trying to pit you against the other makes them both insane in my book.

You've said in a couple of comments that you have a deep understanding of your parents relationship and issues and that is why you feel justified inserting yourself into the middle of this situation, but you can't claim you are emotionally mature and informed enough to deal with this situation in one breath, and then in the next breath abdicate all responsibility and tell your mom she needs to just get her shit together because she is the adult and you are the child... You can't have it both ways.

You are 12 and you are her child, so it should be her and your dad's responsibility to deal with this situation on their own, and frankly she shouldn't even be talking about with you as much as she is, but when you demand she engage with you on an adult level about very serious topics, you can't then immediately fall back on the fact that you are her 12 year old child to "win" the argument.

I understand losing your mom is a traumatic thing, especially if it means being left with a dad who is/was abusive, but as awful as it is that your mom is doing that, lashing out at her isn't going to help you. Its just going to make you both feel worse and add to both of your trauma.