Legally this is not of your concern. You are only obligated by your contract with the original tenant. If the original tenant is allowed to sublease with approval from the landlord that is a matter for the original tenant and the landlord to work out. If you told the landlord your situation you would not be doing anything wrong and it might serve your interests to do so. If you discover that the original tenant is not allowed to sublet, then you could sue the original tenant to void your sublease. It really all depends on your ultimate goal
You need to sign the lease to protect yourself, or find your own place in your name.
For example if the plumbing or electrical fire destroys your personal belongings, and you do not have renters insurance you can't sue the apartment complex as your name is not on the lease. You can't call them to get anything repaired either.
You really need to sign the lease to be covered by insurance by a renters policy, or get your own place.
A guest stays for a limited period of time, not a year or so. Different countries and different U.S. states have different laws, but in most U.S. states you could be evicted since you have no lease, thereby no legal rights. The lessee will probably lose their security deposit, if the complex finds out that he/she is renting out the apartment illegally.
Get this lessee to notify the apartment complex that you were her guest but he/she now wants to move and wishes to sublet it to you. Just pay the additional rent and hope that the complex will allow the lessee to rent it to you. Good Luck
Subletting is legal, but may or may not violate the original tenant's lease.