Why am I underweight and look thin ?

I am 23 years old, weight 48 Kg, height 5'7''. I am thin since my childhood. My parents & siblings are healthy. In childhood, I went to several doctors who told me I was fine without any problems.
I eat meals twice a day and have breakfast & snacks in between. Still why don't I gain weight ? I need to increase at least 15 Kg. Also My hands and legs are so thin ! How can I correct that ?