Scared to ask my stepmom for stuff i need?

im gonna be moving out in a couple months and im gonna need my birth certificate and social security card but my stepmom has them put up somewhere. ( neither she nor my dad know that im moving out yet) i was planning on asking a few weeks before i moved but im scared she wont give them to me. my big sister says she has to and cant keep them once im 18(will be 18 before i move out) and if she doesnt give them to me,i call the cops? is that really wat i have to do? i dont wanna leave on a bad note