Boy cat stalks & mounts sister cat when she walks to litter box?

Hi all. I have a 10 year old boy cat, we adopted him from 2 years old at a shelter, and that's when he got neutered as well. I've heard there's some lingering behavioural issues if you don't neuter them young, but his behavior toward his sister kitty didn't used to be this bad...?

She's his same age, spayed at 9 months, and was already established when we brought him home.

At first, he slowly developed a habit over the past years of temporarily pinning her down by biting her scruff, but stopping once he'd get his name called reprimandingly.

In the last few months however, it's since escalated to stalking her any time she makes her way to the litter box, pinning her down and mounting/humping her before she can even get to it, and not stopping until I physically approach and shoo him away. Same thing if he hears her using the litter box, he'll be waiting right outside the restroom door to pin her.

She never fights back, just kind of sits there and growls unhappily. I'll admit I'm really dumb about inter-cat behavior and don't know what to do here. Is it a dominance thing? A sex/heat thing? Is he stressed and taking it out on her? He used to treat her more like a sibling or littermate, but maybe that's projecting human behavior on them 😅

He also does it whenver he hears her meowing to get into someone's closed room, which means a lot of this is happening at night when I'm asleep and can't get up to shoo him. I feel really guilty for not knowing how to stop or discourage him, and also upset that my poor girl is having to go through this. Advice pls?