I know what 嫂子 means literally, but is there an alternative meaning for it? For example, it's like a "title" I suppose when a person doesn't know their name and just calls them that.
Well you are partially right. 嫂子 can be used to call someone, but it is not a prefix. It is similar to the word "bro". When you use it, you need to know/see the husband of the lady. Using 嫂子 properly can bring you closer to the couple. Don't use it when you see or know the lady's husband is obviously younger than you (it doesn't matter whether the lady is younger than you).
Sister-in-law, who (she) is your brother's wife.
In a broader usage, people call a male friend of similar age as brother, Ge1 Ge1, and call his wife as Sao3 Zi.
A male person avoids to use Jie3 Jie3 or Mei4 Mei4 to call a female of purely friendship, since Jie3 Jie3 or Mei4 Mei4 may be used for their lovers. But a young girl may not like to be called as Sao3 Zi, since Sao3 Zi is more suit for a married lady.