How can I help my mother?

To put it simply, my mother is falling into depression. I don’t know when this started or how long it’s been going on, but I want to help her.
I understand how she feels right now. She thinks everyone is against her and they don’t understand what’s happening to her. I do, though, and it hurts me whenever she and my dad start arguing. She doesn’t think anyone understands her and that they don’t listen when she speaks. My mom truly believes that we don’t love her or care about what she says.

I want her to know that we do love her and that we do listen when she has problems, but she doesn’t think she needs to talk to me (of all people) about it. I know what she needs, but nobody else does and it aggravates me. My dad doesn’t think I know what’s going on and he doesn’t know that none of the things she says is completely against him, so he just argues back. I’ve talked to him about it, but he just blows me off.

I don’t want my mother to hurt like this anymore, I know how serious this is. How can I help her get through this? How can I get them to listen to her and me? It hurts to know she’s hurting.