You are so lucky to have such a wonderful shrub!* You can prune it after flowering if it looks scraggly to you.* They can be purchased from higher end nurseries, but the fragrance is variable, so if you ever purchase one (there is also a yellow flowered variety), be sure it's in bloom so you can select a fragrant one and enjoy the fragrance.* Another reason you are lucky!* Even growing them from seeds doesn't always produce fragrant offspring.
If you see sprouts from the parent, dig up a few and transplant them.* They do best in soil that is rich in organic matter, so add lots of compost and mix it in well to the entire planting area, not just the planting hole.* A 3" or 4" layer of compost mixed in would be good.*
They grow taller in shade.* In the wild they tend to look unkempt and scraggley.* I suspect that is how yours looks.* Can you post a picture?* Maybe I can give you some guidance on pruning to make it look better so your sister can apprecate it too!* Best to prune after flowering.
You could also collect the seeds in December, sow them in pots and sink the pots into the ground for the winter.* You should get some to sprout.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.