First of all you cannot let her brother pull it out! He is big enough to be told "NO" and he will quickly pick that up and do what he is told. He will have to learn the meaning of that word as he grows and becomes more mobile anyhow. Just redirect his energy and say--firmly--"No", babies are smarter than most of us realize. No need to lay a hand on him, except to move his hand away. But don't tolerate him pulling out her hair. That is painful and sends her a horrible message about her importance in the family.
I would not take her to the doctor for this. Wait until you go for a check-up or if she is sick and ask at that time. Sometimes kids don't get hair for a while. My kids are usually bald-ish until about two. (I have 4--12 years down to 11 months) Meanwhile, that "Mane and Tail" shampoo and conditioner with the horses on it...they do sell it on the regular human shampoo aisle, will help. I have really thick and fast growing hair...but I have friends who swear by that stuff for thickening and growing hair.