That is an interesting question. I'm not sure, but I would think that when a kitten begins to explore her environment, at about age 4-5weeks, they can become territorial.
They might not be able to defend that territory, from intruders, but they will still hiss, and posture, and run away. Some may even spit, and lunge at you.
But since you have a 7 week old kitten, and want her to grow up, like sisters...then perhaps you should ask around for 'foster kittens' or other shelters that have kittens, and find out if you can foster, until you are ready to adopt. If you cannot afford one, then perhaps fostering could help with vet costs, and food costs and such. (I'm not sure, but you would have to ask.)
Your kitten would benefit from having a playmate, to teach each other rules of play, and kitten 'manners', bite inhibition, and how to be social with other cats.
But be honest with yourself, and look at what your finances are, and what the extra expenses would be towards having another cat.
Here, young kittens need to have their kitten vaccinations, too, so that is another thing to think about, when wanting to adopt. Plus spaying/neutering.
Don't worry so much about your kitten becoming too territorial, or not being able to adapt to a new cat/kitten...since there are good 'cat to cat introductions' steps that you can take, to make both cats comfortable in your home. Even sister cats don't always get along, perfectly. So it matters more on the personality of the cats involved, versus their age. That is another reason why asking shelters and rescues about a cat's personality is important. They would help you decide.
Just make sure that you socialize your 7 week kitten by giving her a lot of your time, playtime, feedings, brushing, grooming, patting, etc.
And through interactive play with your kitten, teach her about not biting any human's hands/feet/body parts.
You'll have to be the 'kitten social teacher' to your cat, because she is so young, but she'll manage to learn daily through play, and interacting with you.
I hope some of this helps answer your question.