Dear Jecky:
I wonder if you live with your mother and her husband or with your father.
You mentioned a concern that your father will be upset if you bring up a topic to him. When he gets upset how does it show?
If your father is not abusive, hasn’t been abusive when he is upset, or otherwise, I suggest you tell him what you think and what you feel clearly, honestly and respectfully. Don’t tell him to not marry this woman, tell him instead how you feel at the thought of him marrying her. Then listen to what he says. You can post here about such a conversation with him, or about any conversation you already had with him and I will give you my input on it, if you want.
anita