teasing at my house

Hi, I introduced myself and my family some days ago (more details on my website). Lately there has been more and more quarreling between my youngest dd (Lauren) and my stepson (Michael) and it's mostly Laurens fault. She will turn 9 in 10 days, but she's a very tall girl (taller than her teacher!). Her stepbrother on the contrary is really tiny and thin for a 9th grader. It's just genetics I guess. His dad (my DH now) is half a head shorter than me and really thin. His mom didnt even reach five feet. So Lauren is a real tomboy and she likes to show off she's strong. Not a very girly thing I guess, but that's the way she is. Sometimes she just starts pushing her brother. He pushes back but she easily gets him down. Yesterday I heard some noise from her room. I checked it out and saw she was having him pinned on the ground. Off course I got very mad at her. But an hour later she already seems to forget that! I have been speaking to Dh about it. He's after all his father. But he says not to worry about, just kids playing. If it was up to him, he'd hardly interfer.I've also been thinking it would be good for Michael to get some muscles or take some self defence classes. However he hates any physical activity. He rather plays computer or video games. So what do you think? Should I be stricter on Lauren? I guess she isn't meaning it badly. She just doesn't realise she's hurting her stepbrothers esteem. Or should I try to persuade DH to get Michael into sports despite him hating it? It's not that I expect michael to become a big strong guy or so. My girls father was 6'7 and even I am much bigger than his dad, so he'll alway remains smaller than his sisters. However my heart hurts when I see what a pushover he is for Lauren (maybe that's my failing) ps. There are never any problems between Michael and Mallory (6th grade dd). They are best pals.