The fact is, you really can't make your mom better at marriage. You can hope for a better marriage for her this time. I know it must have been hard on you two kids to have had different men being your father. And I bet you tried to do your part in making each marriage be a good one. But, try as you might, you girls can't really make any relationship your mom has succeed or fail.
Some groups also exist for stepfamilies.
It sure would help if you girls had a grownup you trusted and respected to talk to about these feelings. I'd sure need to talk about them if I were you. I want to file for divorce, but I'm afraid because I don't know how it will affect my daughter. My husband and I share custody of our boy.