Why are my bones so thin? will they grow bigger and wider? no one in my family has this except for me and im younger sister

Probably not much: given your age. At 18 y.o. your bones are probably close to the end of their growth. As you know, Vitamin D is very important (along with calcium) for bone growth, so I would make sure that you are getting plenty of each. You may want to ask your doctor for a Vit D-25 test to make sure you are not deficient. Weight bearing exercise is also important for bone growth, so be sure to exercise too!