In reply to phja:
> Any advice would be great.
Disclaimer - long since retired from giving advice, so seek proper advice. Please consult a solicitor and do not sign anything before that.
First rule of thumb is never act as guarantor unless you know and accept fully what the contract says, and you can actually afford to pay if they come after you. A guarantor is exactly that, and normally you can not limit your liability to less than that of the person you are guarantor for is liable in law. What if say you fall out with your family member or vice versa, if they fall out with other tenants, etc, then do you accept that you (most likely) are liable for potentially the whole rent and any and all other tenancy conditions? Bear in mind that your liability will not doubt be open ended in time also, and you, as a guarantor, will not be in a position to end the tenancy yourself.
In this case from what you have said, walk away. A broad brush generalisation, in Scotland/England, it was normally joint tenants were each jointly and individually responsible for paying the rent. Can't imagine things have changed much or that the agreement restricts liability to only the tenant's individual share. You haven't seen the agreement so you have to assume the worst case which is probably they can ask your family member to pay part or all the rent if any one, or more, or all of the tenants default, and if he/she can't or won't pay, they can then come after you.
Plenty of stuff on the web, one example is
https://www.citizensadvice.org.uk/housing/renting-a-home/student-housing/st... . Also, check out Shelter's website for the country you are in. However, do take legal advice as the cost will be small compared with a worst case situation developing.