How would they have known it was Sophie? They never saw her before, they where trying to hide these family, friends, neighbors, etc... walkers. As far as they where concerned they where just more sick people.
Their seem to be a lot here whom don't care about the story telling at all, they just want zombies and zombies getting killed. There is so much good underlying story, acting, and foreshadowing it's unbelievable. I find the subtle things more interesting than anything. Killing tons of zombies over and over will get nothing but old, and cause the series to basically become non existent after a couple seasons.
Well Sophie is a little girl lost in the woods and they found a little zombie girl in the woods. Before Otis died, it was Otis who took the zombies in. So it could be that the other members of the pack did not know about the girl zombie and only Otis. Not sure if they talked about Sophie to Otis before he died.
Reading the comic, and I have to say, the first season improved on the comic alot in my opinion. There are some similarities between the comic and the series but thats about it. There are different characters, some in the comic and not in the series, visa versa.