there are thousands of cons - i would say DONT DO IT! ( i will list some)
1) do you like your family member - if so NEVER mix that with money - if it ends in tears you will have lost a relationship
2) even if the contract is only for six months, if the person extends that you will still be liable until they move
3) if they dont or cant pay, you will have to including being taken to court, getting ccj's etc. It will be like you you live there, but with none of the benefits oof actually living there
Seriously, if you arent able to tell them you dont want to, just tell them you dont have a good credit rating yourself