When a new partner comes on the scene, acceptance is often difficult for children from the first union - regardless of age.
But absence of acceptance should never mean absence of good manners and respect.
Were I in that situation, I would tell my daugher that no-one will ever replace her mother, and no-one is trying to do so.
Nonetheless my new partner is my choice, and I expect the same level of respect for her that you show to me, and if you are unable to do that, then we are going to have difficulty moving forward.
I am not asking you to accept her as your 'new mother', but I am asking you to accept her as the woman I love, and I expect you to employ the manners and courtesy I raised you to have, to be civil in her company.
If you think that is too much to ask, then we are going to have to have a very very difficult conversation about where we go from here.