Oh dear god this.
Nice when others are around, a shit when they're not.
It's not you, he's being horrid.
I do think you need to face that you might lose your family of birth. Come to some sort of terms with that possibility.
And then, -talk- to your mother. Tell her what's going on. Tell her it's been going on for a long time and you're gutted. That he's not being honest.
if she dismisses it, and I'm afraid it seems likely, if she makes out again you're oversensitive, then move away, get the message and let them make the running. Make your own family out of your friends, as far as that is possible. it does help a lot. Keep contact on a lower level and as and when you want it; face full on the pain that your mother is not there for you, and weather it.
The fact is that probably your stepfather is spinning things to make it seem you['re being unreasonable when you're not. Tell it as it actually is, once. Stand your ground. But then accept what comes out of it, and if necessary find a family of choice, not chance.