Ruth isn't Boaz's daughter; she's his servant. So when he addresses her as "my daughter," he doesn't mean it in the literal sense of the word.

At that time and in that place, the word daughter had many different connotations in addition to its literal meaning. Depending on the precise context, a "daughter" could be a sister, a granddaughter, or a descendent. The word could also be used to describe women in general or women from a particular land or city.

Some of the more inattentive of Bible readers have felt decidedly uncomfortable at Boaz's constant references to Ruth as "my daughter." As he intends to marry her, all this talk of Ruth being his daughter sounds a tad incestuous. But although the two are related—distantly, it should be said—there's no concern of their forthcoming marriage violating prohibitions against incest.

In any case, it is important to acknowledge that it was customary at that time and in that culture for an older man or male authority figure to address a younger woman as "daughter." It wasn't intended to describe an actual blood relationship; rather, it was an expression of kindness, and that's primarily how we should interpret Boaz's use of the term in relation to Ruth.