Good that you are so aware at your so young age but it will be wiser if some doctor sees you physically and takes an x ray
of lower back to remove or confirm your doubts, more so, since you complain of pain in lower back since 4 years old, and family history also ,in sibling.but be calm, spina bifida
is harmless unless some nerves are implicated in severe activity. All this is not in your case and mostly spina bifida is without many symptoms for years together , even for lifetime unless some major injury
upsets the region.