Even though you "own" your apartment, the relation between you and the Co-op is that of a "landlord-tenant" relation and is controlled by the "Proprietary Lease" you got when you putchased the apartment. Read it thoroughly and carefully. It typically has an occupancy clause and a sublease clause. Proprietary Lease may vary from Co-op to Co-op but generally have some common occupancy elements allowing your immediate family members like siblings, parents, grand parents, children, grandchildren to live without any formal approval. However, you may still have to notify the Co-op in writing.
But, if you are actually "subleasing", meaning actually collecting rent from those family members residing in a Co-op where subleasing is not allowed, you may get in trouble. Proceed with caution!