It's a pity that someone in the family didn't apply for legal guardianship, or take the poor child into their care before he or she went into foster care, but if you've thought this through properly, and the child's parents have no objection to you applying, then you need to speak to someone at Social Services asap. It can be quite a long-winded process, as the SS have to make perfectly sure that you'd be suitable as parents. As pinktwink says, it'd be a life-changing decision for you, and could cause problems within the family later on, but good luck to you.