Answer:
you and your dad have fast metabolisms.
it may be too hard for you to gain weight.
i'd suggest trying to build muscle. go work on weight machines and eat lots of protein. whey protein really works good.
Talk to a doctor
It's not a disease, just genetics. You can make your arms and legs bigger. You need to start working out in a gym. Do squats and other exercises for your whole body. You will start to fill out. Good luck.