By the age of 17, the growth of the skull bones reach their adult size and proportions. The anterior part of the nasal septum is made up of cartilage which has a better blood supply and hence usually grows faster than the surrounding bone. This causes the septum to bend in the restricted space and results in DNS (Deviated Nasal Septum).
However, DNS is common and is present in more than half the population. The DNS can cause changes in the turbinates (bones) situated on the side (lateral) walls of the nose which in turn blocks the openings of the sinuses.
There can be other causes of persistent nasal blockage like persistent Adenoid tissue, Vasomotor (Intrinsic) Rhinitis, Allergy. Fungal infection, Nasal Polyps and so on.
Decongestant drops should not be used for more than two weeks at a stretch as they have a rebound phenomenon which leads to addiction and causes Rhinitis Medicamentosa.
Steam inhalation is good.
Your daughter needs a more detailed examination to identify the exact cause of her suffering.