I’m so, so sorry your are in this situation. It is very scary. I’m very glad you wrote to us here at PsychCentral.
What you should do — immediately — is tell your mom. Your stepdad’s behavior is not normal. It is not appropriate. It is, in fact, dangerous for you. He is behaving as abusers often do — flattering you, saying sexual things to you that he has no business saying to a 16-year-old girl (especially a girl who is in a daughter role) and then threatening you with the suggestion that if you tell it will hurt your mom.
Don’t take on blame if your mom breaks up with this man. It is not your fault that her husband is sexually harassing you. It is his fault. Her first job is to protect you.
If your mom doesn’t kick this guy out immediately, you need to tell your school counselor what’s been going on. If you think it will be hard to talk about, take this letter to show her.
No kid should be afraid in her own home. No kid should have to deal with “grooming” behavior by her stepdad.
Please don’t wait to get an adult involved. You are in a very vulnerable position.
I wish you well.