Today I found out that my mother is coming back to live with us after leaving my father a few months ago, I'm honestly not sure what to think about it.

So let me start by giving some back story. I'm 16, and I live in Oregon with my parent at the moment. Three months ago my mother left me, and my father, taking my sister with her off to Missouri.
The reason she left was because she had be talking to a guy online for nearly a year, more or less having an affair. Me, and my sister knew about it, about a month or so after it started. At the time we found out, we made a plan
for us all to leave my father. You see, my father was hardly a father to me. I've talked to him more in the past few months than most of my life. He verbally abused me, my siblings, and my mother countless times, made threats to kill
us, leave us, etc. He was just generally an extremely angry, pessimistic, vocal person who made everyone around him feel like shit. This is why we (My sister and I) were happy that we were going to leave and "Start a new life", and we were going to get away from such a negative atmosphere.

So lets fast forward to a few months ago, the day she left. My mom brought some stuff from storage telling me that she needed to get rid of some stuff, and put some stuff we wanted to keep with us to put it in storage so we could get it later. This didn't make sense to me at all, I constantly asked why, how it makes sense. When it didn't. If we were going to move accross the country putting stuff in storage in this state would be stupid. But "Whatever" I said, and I did what she asked. She had drove my dad to work this day, as we only have one working vehicle, that's how she was going to leave. Now at this time, I had no idea she was planning to leave. After helping them a bit she said that I can stop, and was done. So I went in my room and did what I normally do. Turn on some music, and shove my face into a computer screen. She came in and said that she was going to pick up my dad, and that she was bringing my sister's dog with them. I said "He's not going to like that," then she said "Well too bad. Bye, I love you." I said sort of a shitty "Bye", being sidetracked with what I was doing, thinking you know, she would come back.

Later that night, about an hour or so later, I got an IM from my brother saying that my mom left, TEXTED my dad, saying that they were through, and that she was leaving him. At first I was like "Whaaaaaat?" and then I got this knot in my stomache, I got up and looked in my parents bedroom to see if my mom's laptop was there. Obviously enough, it wasn't. She had actually left, without telling me, lying to my face constantly, with the only working vehicle, she exploited my trust. I was fucking crushed at that moment, I didn't know what to think. I was so sad, and alone. I had no one. Anyway, my Grandmother had to come and take my father home that night. My Dad got home, and needless to say he was fucking more crushed than I was. I don't think he slept for three straight nights. I was the only person there too. So I was the only one there for him to talk to, he was just so sad, it was so depressing. I actually felt deeply sorry for him, after legitamently hating him for so long. We talked more in that one week then we did in the rest of my life.

After a few weeks, things got better. My dad was more functional, and working again, things were alright. A few weeks after that my sister was driven back to Oregon to live with us, since my mother realized she can't afford to give her a decent life. Things have been going well, and my dad has been becoming a much better, and more responsible human being. Before, he was addicted to pain medication that my mother was perscribed to, He's been clean for two weeks now, and he says he's been feeling so much better then before. He's fixed his credit, he's happier, more optimistic, he finally fucking smiles, etc, etc. My mom leaving was NOTHING but good for us, nothing

So now I get to the news. Basically instead of my dad getting angry at my mother, and hating her. He tried to stay her friend. They have been talking recently, and apparently she agreed to come back here to live with us, since she realizes that she was fucking retarded to do this in the first place, she realizes that the guy she was chasing is a complete dick, and finally she realizes that her life was just better here.

So, I get this news. My sister is very elated, so happy. I was really glad to see her so happy too, it made me happy. But for a while now I've just been thinking about it. I really try to stay optimistic, but I can't help but think that with my mom coming back, she'll fuck up my dad again, and drive our family to dysfunction again. Of course I miss her, she's my mother, but we're finally starting to become a real family too. I dunno, maybe I'm wrong, maybe she'll change, maybe we'll all be happy. I'm just worried for my dad, and my sister. Hell, I'm not going to be living with my family for too much longer most likely, im just worried for everyone else.

There's honestly a lot more to this story, as it's practically the last 8 years of my life or so, but I wanted to obviously shorten it up a lot.

TL;DR: My mom very selfishly left my dick of a dad for some dude in missouri, leaving me here alone with him, my dad becomes an actual father, tons of good things happen, now my mother who abandoned me wants to come back to live with us.

I'm honestly not sure if the way I'm feeling is right or not... Am I right to not forgive her for what she did? I don't get how my dad, and sister can forgive her either. I get that you should give people leeway for mistakes, but this is just crossing the line in my book. I really just needed to vent a bit, so, thanks.