The section 17 explicitly gives one the right to accommodate (even long term) close relatives:6. Handing over the apartment to another person
The Tenant living in student housing may not, without the permission of the Lessor, hand over the apartment or part of it, or have persons living there who are not covered by the tenancy agreement, or transfer the tenancy agreement. The Lessor requires that the apartment be used for student housing (AHVL 17, 18, 44 and 45§).
(that is the full text of section 17 that they refer to without actually explaining how is their text in anyway compatible with it)Section 17
Assignment of a residential apartment to the use of another person
The tenant may use the residential apartment as his or her own home or as a joint home with his or her spouse and the children belonging to the tenant’s family. The tenant may also use the apartment as a joint home with his or her near relative or a near relative of his or her spouse or assign by sub-lease or otherwise no more than half of the residential apartment to another person’s use for residential purposes if this does not cause the lessor significant inconvenience or disturbance.
http://www.finlex.fi/fi/laki/kaannokset ... 950481.pdf
section 17 is also binding regulation that can not be restricted by terms of the rental contract
text in Finnish: http://www.finlex.fi/fi/laki/ajantasa/1995/19950481Section 26
Prohibited stipulations concerning the use, condition and maintenance of the apartment and liability
Any stipulation restricting the tenant’s rights under sections 16-25 to obtain possession of the apartment, to use it, to assign the apartment or part thereof for another person’s use, to require that the apartment be in the condition laid down in law or agreed, to rescind the agreement or gain exemption from payment of rent or a rent reduction, or to obtain compensation, or which makes the liability of the ten-ant, or of the person to whom the tenant has assigned the apartment or part thereof, for any damage to the facilities designated for residential purposes greater than that laid down in this chapter, shall be null and void.
I dare not to give you a recommendation, but be at least aware if you decide to go ahead and get into trouble, HOAS legal position may not be that strong as they like to claim.