Percy
Take your cues from her, love. If she seems to need a father, be a father as much as you can. If she seems to want something else, because it's too weird for you to be her father that young, try to fill that role.
Lily's more like a friend than a granddaughter to me, but that was simply the way our relationship played out. When Harry was here before, I was more motherly to him. With an older version of him here, it's harder to fit into the 'mother' role because he is older than me.
Just try to relax, and try not to be awkward about her. I know it's easier said than done, but as long as you acknowledge the relationship, you should be all right, however things work out.