her hair is long is correct
As a question, It would be Does your mother have long hair? As a statement, it would be Your mother has long hair.
"What colour is your hair?" is correct.
Your hair is black.
She has curly hair is correct.
Hair is the correct spelling
Yes, they can. It all depends what the person who creates them want to have. <- no. No mermaids do not have hair. Mermaids actually lost there hair when they evolved into mermaids. Hair slows a creature down that is trying to swim. So through evolution these mermaids lose there hair. In conclusion, no mermaids do not have long hair. They have no hair at all. (if you're talking about real mermaids this is correct if not look at the answer before this paragraph.)
Those might be called fringes.
That is the correct spelling of the plural noun "hairstyles."(The person who styles hair is a hairstylist or hair stylist.)
long hair
No, hearing aids cannot correct the destruction of receptor hair cells.
Long hair is in.
A girl's hair is long because the girl made a decision to have long hair, just as a girl with short hair or a boy with short or long hair decided to have that length of hair.