The definition of co-habitation is two or more people of the opposite sex living together under one roof; who are unmarried, boyfriend-girlfriend, engaged couples, or just as roommates,
even if they are not romantically involved it would still be co-habitation.
It is still co-habitation even when it involves friends and siblings, it would be very immodest and inappropriate to live with an unmarried sibling of the opposite sex, people will assume there is more going on than simply trying to save on rent and more often than not they do not know that the other person is a sibling.
Co-habitation is fornication, it goes against God’s laws and commandments and therefore it is a mortal sin, we cannot separate co-habitation and fornication they go hand in hand, they are both mortally sinful and displeasing to God.
so in answer to your question, yes living with someone of the opposite sex, family, friend or not it is still co-habitation and therefore is fornication.
God bless