To give a child of 12 years old the power over you to make you think of leaving your husband is terrible. It's different if she was getting abused, but arguing is normal and, like I said before, be nipped in the bud. She is having far too much influence over her mum's marriage. Would she be happy if they split and then her mum was unhappy single for years to come? She has been happy with her step dad up to now, it's the hormones kicking in and that would happen with a teen's biological dad too, not because he is her step dad.
Holdthetonic Are you unhappy in your marriage, personally, apart from this with your daughter? Because if you are happy enough, then you have to ride this wave. She will be stroppy no matter if you are together or on your own but at least still married, you have someone to confide in, even if he is having trouble himself with dealing with her. They need to find some common ground and leaving them to it sometime might do them good instead of being there to be the go between all the time.
Just a note, my children have never wanted me to be with anyone else since my divorce, even though my ED didn't want me to be with her dad anymore. I have been single now for over 10 years. Now I am living on my own a romance would be nice but I have put it on hold for a very long time. My son would still not be too happy but it's hard luck now, it's my life and if I want someone special in it now, then I will do. Children cannot rule what we do. We can take their feelings into consideration, but ultimately, as long as they are not being abused, then they cannot tell us how to live our lives, it is far too much power, in my opinion.
My parents were at loggerheads most of their marriage but it wasn't my decision to make to have them split up.