How can I cope??

My adopted Mom since I was 10, has befriended my brothers ex, and this girl is psycho, claiming she is pregnant, telling others she can't have any kids. We (my daughter and I) have e-mailed this girl and told her of our feelings, some e-mails were not very nice!! This girl has showed them to my Mom and now she isn't talking to me or her granddaughter, who is now going to have a baby of her own, and my mom does not care!! This girl has a 12yr old son and had him call my brothers now girlfriend, and he proceded to call he a , slut and even said my brother had a small . What mother would have her own child do that? But when I tried to tell my mom what she does, she asks me where my compion is and that i should help this girl and feel sorry for her...NO WAY!!! But my thing is I am no longer wanting to have anything to do with my adopted mom or this girl..My mom chose this person that she's known for 1 1/2 yrs over my kids and me since I was 10. How does someone cope with that?