Divorce: Should I be more angry?

The past few weeks my folks have been fighting. My mom (who I tend to blame more so than my dad) I believe is going through a midlife crisis. I'm not too sure about what that specifically is, but I know its stressed my family and I out to the max. She usually is all put together and steadily praying and reading her bible and leading the family in devotions, but now she is drinking, smoking cigarettes again (she quit 11 years ago), and dropping a F bomb literally every time we get the chance to speak to her. As for my father, he just kind of ignores whats going on, like nothings the matter. But, there is. My fathers lack of caring drives my mom insane, so, as of now he is out of the house for most of the week, and he usually sleeps at our small rent house down the street from where we live. He doesn't understand, and neither do the rest of us, but he just isn't smart enough to leave her be I guess.

I have no idea what drove my mom to whatever is going on with her, but I start school in a week and I find it, surprisingly nearly out of my thoughts most of the time.

My mother always talks to us (meaning my brother and sister) about her issues with dad. I'm fifteen, my brother is seventeen, and my sister ten. Both of them seem to kiss ass, but I never have a problem telling her what I think. She usually uses this against me in most cases, but she also knows I will always be honest and talks to me more than the other two.

Personally, I hate it that she keeps bringing us into her and my dads problems. Especially when my ten year old sister is exposed to all that adult bull ****. I can't handle much more of that, I find it immature, and I think its just a conspiracy to turn us against dad for custody purposes. My dad has said little comments here and there, but never disrespects her as much as she does him.

She is always screaming that she wants a divorce. I know that my dad wants the same thing too, because he told me one day. He didn't actually directly tell me, but he spoke when he knew I was the only one in the room.

I feel a little angry, and when I really let myself think about it, I feel sad.
But overall I feel confused as too why I just don't seem to care. Is that normal?

My parents sent me and my siblings to stay with my grandmother for the week so they could talk things over, and while I was here my grandmother wanted to talk about what was going on. Now, she is a registered psychiatrist mind you, she said lots of different things that made me think way too hard, but one question got to me especially, and it was, "Well how is all this making you feel?"
For the first time, in the weeks they've been fighting, I felt my throat get thick and tears were coming. All I said to her was 'I don't know,' but there was more there. I just wouldn't allow myself to go there. Before she said anymore, I gathered myself up, like nothing happened. Everyday, I wonder whats wrong that I don't feel what my brother does, which is stressed out, trying to figure out why mom is acting so crazy and trying to protect her to dads stupidity, or how my sister does, which would seem more me, which is just simply heart broken over the whole situation. I really don't understand, and I'm speaking to a bunch of strangers, but I really would like your input. Thanks so much.