Kittens at this young an age really are surprisingly lightweight. If she doesn't feel obviously skinny with ribs and hip bones clearly protruding or something like that, she's probably at the right weight.
That said, if you would like to give your baby a nutrient boost, there are a couple of things you could give her. One is a product called "KMR." It's a milk product specifically formulated for orphaned kittens, that comes as a liquid or in powdered form. In your shoes I might be concerned enough about the different calcium requirements of a kitten versus a cat to give the kitten a couple of tablespoons of KMR every night as a side dish with her dinner for about three or four weeks, but it's definitely an extra. Also, you wouldn't want to give too much because since she's pooping appropriately now, you REALLY don't want to give her the runs. (For that same reason, don't give her plain cow's milk or cream. It's hard on a kitty's digestive tract.)
If you get KMR, the liquid form is easier but less practical, because with only one kitten eating it, you wouldn't use a whole can of the liquid before it would not be safe to continue serving from it. (Enriched milk based products are a perfect medium for growing bacteria. Even kept in the fridge, once it's open it's not a great idea to keep it after about three days.) The powder is a nuisance to mix well but has the advantage of safety -- you just mix what you need and save the rest of the powder in the fridge.
Another way (probably easier) to help a kitten to gain weight if you're really concerned is to feed her dry kitten chow along with her wet food. She will get her kitten nutrition that way, since kitten chow is formulated correctly for kittens, and since dry food is more caloric than wet food, she should plump up. Kibbles are a bad idea to feed a cat if he is fat, and some cats do get a preference for dry food to the point they won't eat canned food, so watch that, but giving a small amount on the side would help your baby gain a bit of weight while also giving her the vitamins and calcium she needs.
The other thing is, check her stool for any sign of worms. Vets automatically worm kittens on their first visit, assuming all kittens have roundworms. Maybe they do and maybe they don't, but the meds used to worm a kitten for roundworms are considered safe so they just do it as a matter of course. Roundworm medication can be bought online or at the pet store, just be sure of the doses if you want to worm your baby, since she's still so young. Don't give her anything else, though. (Like, if you have tapeworm pills on hand for your other cat, don't dose the kitten.) You're always better off giving a kitten things like this only on the advice of the vet, but if you can't get to the vet and you actually see worms in her poops, you could try roundworm medication pretty safely if the instructions say she is old enough. Just be totally careful about the dosing.
If the kitten is eating, pooping appropriately, and playing, she sounds like she is doing fine. When you say "her brother," do you mean your grown cat? If so, he's probably not going to catch anything from her unless he himself has not been vaccinated. It sounds like you're doing fine. Six weeks is a bit young to lose her mama, but if she has the adult cat in your household to interact with and that is going well, she will probably come out of the loss of her mother just fine.