Since I am unclear where the situation happens at I can simply tell you how I avoid this issue with my kids & the ones I watch. Every child I watch and each of my own has a small pop up tent. If you spend a little time on searching, you can find decent options at not too high of price that fold down super flat & are easy to store. I current have 7 of them & they all fit easily under my couch with room to spare.
The point of the tent. Every person gets a retreat space. I set them up in out of the way locations. The child can place any item in there to not share. They are not permitted to fill it with items to not share, it's one or two personally owned items they do not want to share. They can use those items inside their tent only. They also have a pillow & blanket if they want. When someone goes into their tent, it's a time out for interaction. No one can bother them, talk to them, etc. No one goes into anyone else's tent, ever. The tent is for alone time. I put my own kids in their room. This has worked very well for avoiding conflicts, particularly physical ones. I even let them go in there if they do not wish to talk to me. They will have to talk to me when they emerge, but they can choose a cool down time before we talk. I should mention I do not allow electronics inside the tents. I am pretty sure if I did some might stay in there all day.
So I tell you all that in case this might work. It would allow your son a space to say "ENOUGH", but since she would also get one, that she too would want respected, she may see the barrier clearly enough to back off when he tries to walk away from her. Items I bought that I recommend for the kids to stock in their tent are fidget toys (not spinners, there is a while range out there), chew toys (dishwasher safe, so doesn't matter who used it last and yes, frustrated 6 year olds love to chew, remember chewing your pens/pencils or kids who did? Same idea, only safer since it's silicone, better for teeth & health), squeeze toys (the type for frustration) and other similar items. I have items in a big bin, many identical items so every kid can pick something to chew, squeeze and so on. They can choose items from that for their tents to help them calm themselves, including simple things, like we made water bottles together with oil, water & food color, we also made some with glitter or other things. Those calm down bottles have all kinds of tutorials online. I would recommend gluing the caps in place. Believe me, if you do those, just glue it. You would thank me for that tip if you knew the misfortune I had with a toddler and one of those. ;)
If this conflict is happening at mom's house. You can buy a tent for him for your house, and start using it. Then you can tell mom how great that has been working and offer to provide tents for the kids to use there since your son loves it so much & you don't want to leave the step sister out. You can then also have the son gift the tent to the step sister and try to pick one you think she will adore. It could garner some much needed goodwill too, as the best case scenario is that she may one day want him to like her & as such, act in ways that will inspire that bond. If you are gifting, you could also have him help make calm down bottles & such so that they both get items for their tents that are for helping them settle down & rebalance before rejoining interactions.
Here is one example (with directions) on a calm down bottle that my kids LOVE, yes even the boys. Falling glitter is mesmerizing. http://mskcpotter.blogspot.com/2015/06/calm-down-jar-upgrade-and-recipe.html
You can also use distilled water by itself & colored paperclips. I added a couple caps of peroxide because I was still worried about it going yucky. Anyway, you can then tie a magnet on a string to the lid & use the magnet to drag the paperclips all over from outside the bottle. Mine also love that one a lot.
Add on since I see now the situation is happening on your own home. That is better anyway, since you can have more influence in your own home than you can elsewhere. So in my home we make rules together as a family, the kids are really prompted to come up with them, generally everyone easily agrees (like you can't go into someone's else's room without permission, no hitting, etc). You can set up rules that help cover this issue & you can prompt with questions like "What about when...." and see what they come up with rules on their own. Then when a rule is being broken, you can also say "hey I thought you were the one who said...". We also all sign the rules contract. In our house that contract is for everyone.
I definitely think mom is off on her message here. It sounds as though she is condoning verbal assaults based on being female. Egging someone on & running your mouth isn't okay no matter the gender of who is doing it, same for hitting. And this idea that being smaller means you can be easily taken down isn't reality either. I am about half the weight of my husband and a foot shorter & he will tell you in wrestling, etc that I am much harder to tackle than many people larger than me. It depends on if you have a lot of spunk, little concern for pain or a high pain threshold and if you know what you are doing. I am not saying it's right, but when I was 5yrs old I gave one my teenage siblings a black eye, on purpose (I hit on purpose, the black eye was more than I planned on). Being smaller doesn't mean she has license to get away with hitting nor does it mean she isn't capable in inflicting serious pain. I tell my kids they are permitted to use enough force to get away in a situation where they are being hit. It's not been something that we've needed to use, but I feel it's a reasonable thing to say & I can't tell them to never hit anyone ever. That isn't how life goes sometimes. I do want them to avoid it when it's possible though & it is usually possible to avoid.
So I do think you need to work on what messages your children are getting, that they are strong, firm, confidence building & limit setting. It is not okay to tell a boy that girls are smarter. It is not okay to a boy that boys are stronger. Those are far from universal truths & they do not help in character building for either kid. What I find helpful in my kids is I speak what I want to see. I say things like "I know you have a loving heart. What is going on right now that you can't show that part of you?" I find it much more effective than saying "Why are you being such a little meanie pants?" Your step daughter, at 5, isn't some awful diabolical little tyrant as much as it may seem so. She has reasons for doing what she does & when you sort out why she is doing those things, often solutions are much more attainable. Kids will act it out when not happy. We call it "acting out" instead. We also say things like the kid is "giving" me a hard time instead of seeing the kid is having a hard time. Children don't just up & decide to be aggressive. I didn't give my sibling a black eye for no reason. If you think back to your own childhood & the things you did, there were reasons you did them. If those had been resolved ahead of time, chances are you would have never misstepped. It could be something like jealousy even, especially if she doesn't see her dad often, or it could be insecurity if she thinks her mom might love him more than her, or or or. Children usually have something under the surface going on when their behavior seems this level of purposeful & repetitive & unkind.