That 4th person (sister) needs to sign the lease. Yes, your landlord can take legal action and evict all of you. To avoid this, get that 4th person to sign the lease pronto. Even if your family member never told the landlord about this person, the neighbors will complain and notify the landlord. Much easier to get this 4th person to sign a lease than having complaints from the neighbors. For your information, neighbors have much louder voices than any member of your family does. They go directly to the county and file lawsuits.