Inquiry about thinning hair

Given what you are saying, it sounds as though you may be dealing with multiple problems here. First there is the issue of your hair’s condition. This is something on which I can advise you. Second, it sounds as though you are experiencing either breakage or hair loss at the top of your head. If it is breakage, again, I can advise you. If it is hair loss, I suggest you visit a doctor and discuss the matter with him or her.
Let’s look at the second issue first. In the area where your hair is “very thin, as if I am going bald” do you see broken stubs of hair at the scalp? Or is the hair just thin there? If there are no “stubs” then you don’t have breakage, but most likely some form of hair loss. While it is possible that the loss could be hereditary, it would be unusual. I am not licensed or qualified to diagnose or recommend treatment for such conditions, so if you are concerned about the possible cause, please visit your doctor or dermatologist.
As for the possibility that it is breakage, this would be handled the same way the rest of your hair should be dealt with given the condition you state that your hair is in. You have obviously over-stressed your hair with heat styling on top of frequent color processes. The first thing I suggest is to begin with weekly deep-conditioning treatments. Apply your favorite conditioner to your hair, comb it through with a wide tooth comb, cover it with a plastic cap and wrap a hot towel (from the dryer) around the hair and leave it for twenty minutes. Rinse the conditioner from the hair with cool water.
Avoid heat styling whenever possible, and do not use ponytail elastics or barrettes. If you wear your hair up, be sure to do so loosely. Don’t do anything to add stress or tension to the hair if it can be at all avoided.
Be sure to use protein-rich shampoos and conditioners to help strengthen the hair. Also, using extra conditioning products like leave-in conditioner sprays and anti-frizz serums to smooth the cuticle layer are great to prevent further damage. As for the suggestion of taking vitamins: this is not a bad idea. Vitamins will rarely do anything to correct hair loss (if you are losing hair due to vitamin deficiency, you will have other serious symptoms that would alert you to the problem faster than hair loss), but good nutrition is key to growing healthy hair.
The best way to determine what the problem most likely is would be to compare the way you style and treat your hair with the ways in which your sisters treat and style their hair. Look for things you do differently. I’m sure if you sit down and make a list of what you and your sisters do differently, you will soon discover where your styling habits may have led you wrong.
The average person’s hair grows at a rate of ½ inch per month. This being the case, you could see healthy, shoulder-length hair in as little as 16-18 months with regular trimming.