Biting "Back"?

Sorry if this has been covered - I'm getting errors when I try to search the forums, and I'm not finding any info on this outside TCS.

Sophie and Jack are still having a really weird dynamic (or it seems weird to me). They have no issues eating together (they even took turns taking catnip off a small dish last night), will sit in close proximity together at times, and will chase each other like maniacs all night long.

However, Sophie seems hell-bent on being a thorn in Jack's side sometimes. She will walk up to him, greet him perfectly calmly (nose to nose), they will even have a lick or two...and then she starts following him, practically walking on his back paws, and nipping at his back, along his spine. Doesn't appear to be hard (no marks, no saliva left behind on his coat), but he reacts very strongly, usually turning and giving her a smack, even hissing. Is this a dominance thing? I've seen her do the classic "hop on Jack's back and nip his neck," to which he submits, but when she follows him around nipping at him like this, he definitely does NOT submit. Oddly, when they do this, she seems to be totally unruffled...aside from that little head darting out for a quick nip every few seconds, her posture, tail position, ear position, etc, all say "it's all good."

Still just trying to feel things out, because while sometimes they seem like they're getting along famously, there are other times when Jack seems really unhappy and Sophie just acts like a bully. I don't want to interfere with them if they're still trying to determine their own hierarchy, but I'd really like to understand all these little quirks, so I can know when it's appropriate to step in, and when it's better to let them be.

Now THAT is the perfect Sophie picture for this situation. A friend actually asked me if that photo was altered to make her eyes so huge...we refer to that as the "bug-eyed crazies." She is only 9 months old now (was about 7-8 weeks in that pic), and Jack is roughly the same age. And yes, she is a total princess. She sashays around the house with that kitty swagger that says "Yup, this is all mine."

I'm glad I'm not crazy to think that some of this was bullying. I've been so worried about upsetting some hierarchy balance by breaking things up, but with these two, it can be hard to tell where the line is drawn, and when exactly it is crossed.

Soph hasn't been very interested in interactive toys since she was much younger - I eventually gave away my laser pen, and Da Bird has been sitting in a drawer. [I actually got Jack in the hopes that he would give her the exercise and activity she needs, and for the most part, he does.] I'm going to give Da Bird another shot though...Jack loves the thing, and like human siblings, she often seems to take a greater interest in things that HE wants.

I'm ashamed to say it, but I think I may have caused some of her aggressive tendencies by simply not showing her enough one-on-one attention. She's not normally a super-snuggly cat, and I've always let her come to me for attention. Since Jack's been around, I've stuck to that, for fear of irritating her more if I "hovered" too much, when she's already stressed out. She was acting exceptionally needy yesterday evening, and so I dropped everything I was doing to just lie on the bed with her for a good long while. After about an hour of petting her while she slept curled up by my side, she actually consented to let Jack join us (although after awhile, he ruined it by taking advantage of her mellow state to jump on HER).

They went off and played "kitty tag" for a good hour after that, just a good game of chase, and at bedtime, she curled up by my head (within petting reach, of course), and she was content enough at bedtime to allow Jack to SLEEP on the bed at my feet (normally he is not permitted to set one paw there without being ejected!).

She was back to picking on him a bit this morning, and he's a bit twitchy around her at this point, but I'm hoping that if I find the right balance of attention, interaction, and giving them their space to work things out, they will smooth things over.