Dry Cell Huge

I am 26 and my daughter is 5 years old she has good hair (wavy hair curls when wet).for almost 2 years now her hair has been very dry i've tried different types of grease,hair lotions,cremes for hair, kids shampoo everything. the last 3 months or so i've notice her hair is really short and now she has a big bald spot in the back of her head. she has a twin sister and her hair is much longer and thicker than hers.there hair used to be the same length.(sister has always been thicker) If i part her hair down the middle her hair look very shaggy. since she was about 2 she used to get very small bald spots but it always grew back before i could get worried. but this is a huge one and it has never been this bad or last this long. i am so worried what should i do? and another thing her nails don't grow either and her sisters nails have to be cut every week. her sister also has sickle cell disease and she has the trait does that have anything to do with it? please help with this