My stepfather's unwanted advances and groping when my stepmother is absent

I've always had a bit of a strange relationship with my stepfather. When my stepmother's not around, he can't help himself from making unwanted advances and groping me. It's always been a bit of a weird dynamic between us, but I try not to let it bother me too much. However, recently, things have been heating up a bit more than usual. He's been getting more and more bold with his touches, and I'm starting to worry that he might cross a line that he shouldn't. The other day, when my stepmother was out running some errands, my stepfather decided to take advantage of the situation. He cornered me in the kitchen and started to grope me again. I tried to push him away, but he was too strong. He forced me to bend over and started to touch me in ways that made me incredibly uncomfortable. I don't know what to do about this situation anymore.